Source code for dateadd.bat

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@ECHO OFF
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ECHO.
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:: Reset variables
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SET "cDate="
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SET "iDate="
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SET "sDate="
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:: Check the Windows version
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IF NOT "%OS%"=="Windows_NT" GOTO Syntax
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SETLOCAL ENABLEDELAYEDEXPANSION
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:: Initialize variable
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SET "Error=0"
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:: Check the command line arguments
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IF     "%~1"=="" GOTO Syntax
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IF NOT "%~3"=="" GOTO Syntax
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ECHO.%* | FINDSTR.EXE /R /I /C:"[^-\""adnotwy/0-9 ]" >NUL && GOTO Syntax

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:: Get today's date in local date format

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SET ArgsCount=0
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FOR %%A IN (%Date%) DO (
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	SET /A ArgsCount += 1
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	SET "Today=%%~A"
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)
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IF %ArgsCount% GTR 2 (
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	REM Use WMIC to get today's date in ISO 8601 format

	FOR /F "skip=1 tokens=1-3" %%A IN ('WMIC Path Win32_LocalTime Get Day^,Month^,Year /Format:table') DO (

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		IF NOT "%%~F"=="" (
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			SET Today=%%C-%%B-%%A
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		)
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	)
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)
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IF "%~2"=="" (
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	SET "cDate=%Today%"
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	SET /A "cDays=%~1" >NUL 2>&1
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) ELSE (
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	SET "cDate=%~1"
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	IF /I "%~1"=="Now"   (SET "cDate=%Today%")

	IF /I "%~1"=="Today" (SET "cDate=%Today%")

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	SET /A "cDays=%~2" >NUL 2>&1
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)
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IF "%cDays%"=="0" (
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	ECHO.
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	ECHO Error: %cDays% is not a valid integer

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	GOTO Syntax
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)
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:: Read the Date format from the registry; if this fails, we're probably dealing with an older Windows version

FOR /F "tokens=3" %%A IN ('REG.EXE Query "HKEY_CURRENT_USER\Control Panel\International" /v iDate 2^>NUL') DO SET "iDate=%%~A"

FOR /F "tokens=3" %%A IN ('REG.EXE Query "HKEY_CURRENT_USER\Control Panel\International" /v sDate 2^>NUL') DO SET "sDate=%%~A"

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IF "%iDate%"=="" GOTO Syntax
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:: Check if a valid date in local format was specified

ECHO.%cDate% | FINDSTR.EXE /R /B /C:"[0-9][0-9]*%sDate%[0-9][0-9]*%sDate%[0-9][0-9][0-9][0-9]" >NUL

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IF ERRORLEVEL 1 (
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	REM Try testing for ISO 8601 format
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	ECHO.%cDate% | FINDSTR.EXE /R /B /C:"[0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]" >NUL

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	IF ERRORLEVEL 1 (
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		ECHO.
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		ECHO Error: %cDate% is not a valid date

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		GOTO Syntax
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	) ELSE (
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		SET "iDate=2"
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		SET "sDate=-"
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	)
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)
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:: Parse the date specified, abort on failure

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CALL :ParseDate %cDate%
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IF "%Error%"=="1" GOTO Syntax
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:: Convert the parsed Gregorian date to Julian

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CALL :JDate %GYear% %GMonth% %GDay%
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:: Display original input
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ECHO Starting date   : %cDate%
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:: Add or subtract the specified number of days

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IF "%cDays:~0,1%"=="-" (
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	SET /A "NewJDate = %JDate% - %cDays:~1%"

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	ECHO Days subtracted : %cDays:~1%
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) ELSE (
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	SET /A "NewJDate = %JDate% + %cDays%"
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	ECHO Days added      : %cDays%
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)
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:: Convert the new Julian date back to Gregorian again

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CALL :GDate %NewJDate%
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:: Reformat the date to local format
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CALL :ReformatDate %GDate%
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:: Display the result
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ECHO Resulting date  : %LDate%
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:: Return the result in a variable named after this batch file

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ENDLOCAL & SET "%~n0=%LDate%"
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GOTO:EOF
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::===================================::
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::                                   ::
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::   -   S u b r o u t i n e s   -   ::
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::                                   ::
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::===================================::
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:GDate
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:: Convert Julian date back to "normal" Gregorian date

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:: Argument : Julian date
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:: Returns  : YYYY MM DD
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::
```

:: Algorithm based on Fliegel-Van Flandern

:: algorithm from the Astronomical Almanac,

:: provided by Doctor Fenton on the Math Forum

:: (http://mathforum.org/library/drmath/view/51907.html),

:: and converted to batch code by Ron Bakowski.

```
::
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SET /A "P      = %~1 + 68569"
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SET /A "Q      = 4 * %P% / 146097"
```

SET /A "R      = %P% - ( 146097 * %Q% + 3 ) / 4"

SET /A "S      = 4000 * ( %R% + 1 ) / 1461001"

SET /A "T      = %R% - 1461 * %S% / 4 + 31"

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SET /A "U      = 80 * %T% / 2447"
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SET /A "V      = %U% / 11"
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SET /A "GYear  = 100 * ( %Q% - 49 ) + %S% + %V%"

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SET /A "GMonth = %U% + 2 - 12 * %V%"
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SET /A "GDay   = %T% - 2447 * %U% / 80"
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:: Clean up the mess
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FOR %%A IN (P Q R S T U V) DO SET "%%~A="

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:: Add leading zeroes
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IF 1%GMonth% LSS 20 SET "GMonth=0%GMonth%"

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IF 1%GDay%   LSS 20 SET "GDay=0%GDay%"
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:: Return value
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SET "GDate=%GYear% %GMonth% %GDay%"
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GOTO:EOF
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:JDate
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:: Convert date to Julian
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:: Arguments : YYYY MM DD
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:: Returns   : Julian date
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::
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:: First strip leading zeroes
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SET "MM=%2"
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SET "DD=%3"
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IF "%MM:~0,1%"=="0" SET "MM=%MM:~1%"
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IF "%DD:~0,1%"=="0" SET "DD=%DD:~1%"
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::
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:: Algorithm based on Fliegel-Van Flandern

:: algorithm from the Astronomical Almanac,

:: provided by Doctor Fenton on the Math Forum

:: (http://mathforum.org/library/drmath/view/51907.html),

:: and converted to batch code by Ron Bakowski.

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SET /A "Month1 = ( %MM% - 14 ) / 12"
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SET /A "Year1  = %~1 + 4800"
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SET /A "JDate  = 1461 * ( %Year1% + %Month1% ) / 4 + 367 * ( %MM% - 2 -12 * %Month1% ) / 12 - ( 3 * ( ( %Year1% + %Month1% + 100 ) / 100 ) ) / 4 + %DD% - 32075"

FOR %%A IN (Month1 Year1) DO SET "%%~A="

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GOTO:EOF 
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:ParseDate
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:: Parse (Gregorian) date depending on registry's date format settings

:: Argument : Gregorian date in local date format, or in ISO 8601 format

:: Requires : sDate (local date separator), iDate (local date format number)

:: Returns  : GYear (4-digit year), GMonth (2-digit month), GDay (2-digit day)

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::
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IF "%iDate%"=="0" FOR /F "tokens=1-3 delims=%sDate%" %%A IN ('ECHO.%1') DO (

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	SET "GYear=%%~C"
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	SET "GMonth=%%~A"
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	SET "GDay=%%~B"
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)
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IF "%iDate%"=="1" FOR /F "tokens=1-3 delims=%sDate%" %%A IN ('ECHO.%1') DO (

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	SET "GYear=%%~C"
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	SET "GMonth=%%~B"
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	SET "GDay=%%~A"
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)
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IF "%iDate%"=="2" FOR /F "TOKENS=1-3 DELIMS=%sDate%" %%A IN ('ECHO.%1') DO (

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	SET "GYear=%%~A"
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	SET "GMonth=%%~B"
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	SET "GDay=%%~C"
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)
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IF %GDay%   GTR 31 SET "Error=1"
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IF %GMonth% GTR 12 SET "Error=1"
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GOTO:EOF
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:ReformatDate
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:: Reformat the date back to the local format

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:: Arguments : YYYY MM DD
```

:: Returns   : LDate (Gregorian date in local format)

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::
```

IF "%iDate%"=="0" SET "LDate=%~2%sDate%%~3%sDate%%~1"

IF "%iDate%"=="1" SET "LDate=%~3%sDate%%~2%sDate%%~1"

IF "%iDate%"=="2" SET "LDate=%~1%sDate%%~2%sDate%%~3"

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GOTO:EOF
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:Syntax
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ECHO.
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ECHO DateAdd.bat,  Version 2.02 for Windows XP Professional and later

ECHO Add the specified number of days to the specified date

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ECHO.
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ECHO Usage:   DATEADD  [ date ]  days
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ECHO.
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ECHO Where:   date     is a "normal" Gregorian date, either in the local date

ECHO                   format, or in ISO 8601 yyyy-MM-dd format, or "Today"

ECHO                   or "Now" (default if not specified: today's date)

ECHO          days     is the number of days to add (negative to subtract)

```
ECHO.
```

ECHO E.g.     DATEADD  2017-01-01   1        will return 2017-01-02

ECHO          DATEADD  2017-01-01  -1        will return 2016-12-31

:: To show the following "advanced" examples, Windows XP or later is required

IF NOT "%OS%"=="Windows_NT" GOTO Skipped2

FOR /F "tokens=3" %%A IN ('REG.EXE Query "HKEY_CURRENT_USER\Control Panel\International" /v iDate 2^>NUL') DO SET "iDate=%%~A"

FOR /F "tokens=3" %%A IN ('REG.EXE Query "HKEY_CURRENT_USER\Control Panel\International" /v sDate 2^>NUL') DO SET "sDate=%%~A"

:: Windows XP or later is required to read iDate, so if iDate isn't set, skip the "advanced" examples

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IF "%iDate%"=="" GOTO Skipped2
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FOR %%A IN (%Date%) DO SET "Today=%%~A"
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CALL :ParseDate %Today%
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IF NOT "%Error%"=="0" GOTO Skipped1
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CALL :JDate %GYear% %GMonth% %GDay%
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SET /A "NewJDate = %JDate% + 1"
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CALL :GDate %NewJDate%
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CALL :ReformatDate %GDate%
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SET "Tomorrow=%LDate%"
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CALL :ParseDate %Today%
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CALL :JDate %GYear% %GMonth% %GDay%
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SET /A "NewJDate = %JDate% - 1"
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CALL :GDate %NewJDate%
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CALL :ReformatDate %GDate%
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SET "Yesterday=%LDate%"
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:Skipped1
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IF "%Yesterday%"=="" (
```

	ECHO          DATEADD  %Today%   1        will return tomorrow's date  ^(as will DATEADD  1^)

	ECHO          DATEADD  %Today%  -1        will return yesterday's date ^(as will DATEADD -1^)

```
) ELSE (
```

	ECHO          DATEADD  %Today%   1        will return %Tomorrow%

	ECHO          DATEADD  1                     will return %Tomorrow% too

	ECHO          DATEADD  %Today%  -1        will return %Yesterday%

	ECHO          DATEADD  -1                    will return %Yesterday% too

```
)
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:: End of "advanced" examples section
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:Skipped2
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ECHO.
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ECHO Notes:   By default, the returned date will be presented in the local date

ECHO          format; however, if the date in the command line argument is in the

ECHO          ISO 8601 yyyy-MM-dd format, the result will be in ISO 8601 format too.

ECHO          Unlike its earlier 1.* versions, this batch file does not require

```
ECHO          elevated privileges.
```
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ECHO.
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ECHO Credits: Julian date conversion based on Fliegel-Van Flandern algorithms from

ECHO          the Astronomical Almanac, provided by Doctor Fenton on the Math Forum

ECHO          http://mathforum.org/library/drmath/view/51907.html

ECHO          and converted to batch code by Ron Bakowski.

```
ECHO.
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```
ECHO Written by Rob van der Woude
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ECHO http://www.robvanderwoude.com
```
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IF "%OS%"=="Windows_NT" ENDLOCAL
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IF "%OS%"=="Windows_NT" EXIT /B 1
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```