Date and Time in Windows NT 4 and later

Advanced Date Math

What would it take to do some real math with dates in batch files: add a couple of days, find out the weekday of 3 months ago, ...?

It would require a way to "linearly" convert any date to a number and back again.

Sounds too complicated?
As a matter of fact, there already is a method to do that.
The "number date" is called the Julian date: the interval of time in days and fractions of a day, since January 1, 4713 BC Greenwich noon (WikiPedia).
The current Julian date would be approximately 2460425 (calculated by the webserver's PHP interpreter, then truncated to an integer value).

Good news: someone has already written the conversion subroutines in NT batch!
Thanks Ron Bakowski for the following code:

:JDate
:: Convert date to Julian
:: Arguments : YYYY MM DD
:: Returns   : Julian date
::
:: First strip leading zeroes
SET MM=%2
SET DD=%3
IF %MM:~0,1% EQU 0 SET MM=%MM:~1%
IF %DD:~0,1% EQU 0 SET DD=%DD:~1%
::
:: 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.
SET /A Month1 = ( %MM% - 14 ) / 12
SET /A Year1  = %1 + 4800
SET /A JDate  = 1461 * ( %Year1% + %Month1% ) / 4 + 367 * ( %MM% - 2 -12 * %Month1% ) / 12 - ( 3 * ( ( %Year1% + %Month1% + 100 ) / 100 ) ) / 4 + %DD% - 32075
SET Month1=
SET Year1=
GOTO:EOF

This code tells me today's Julian date is 2460427, which I find not bad at all: keep in mind that the batch file does not correct for time zone offset, and that batch files cannot handle floating point numbers.
Also keep in mind that on any computer, the errors will more or less even out, so date differences will be calculated correctly.

:GDate
:: Convert Julian date back to "normal" Gregorian date
:: Argument : Julian date
:: Returns  : YYYY MM DD
::
:: 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.
::
SET /A P      = %1 + 68569
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
SET /A U      = 80 * %T% / 2447
SET /A V      = %U% / 11
SET /A GYear  = 100 * ( %Q% - 49 ) + %S% + %V%
SET /A GMonth = %U% + 2 - 12 * %V%
SET /A GDay   = %T% - 2447 * %U% / 80
:: Clean up the mess
FOR %%A IN (P Q R S T U V) DO SET %%A=
:: Add leading zeroes
IF 1%GMonth% LSS 20 SET GMonth=0%GMonth%
IF 1%GDay%   LSS 20 SET GDay=0%GDay%
:: Return value
SET GDate=%GYear% %GMonth% %GDay%
GOTO:EOF

This subroutine, when called with today's Julian date 2460427, returns 2024 04 26.
Not bad at all...

How about some date math? Let's try and find the date of 5 weeks ago (assuming US date format MM/DD/YYYY):

@ECHO OFF
:: Strip the day of the week from the current date
FOR %%A IN (%Date%) DO SET Today=%%A
:: Parse the date, prefix day and month with an extra leading zero
FOR /F "tokens=1-3 delims=/-" %%A IN ("%Today%") DO (
	REM For European date format DD-MM-YYYY use SET Day=0%%A and SET Month=0%%B instead
	SET Day=0%%B
	SET Month=0%%A
	SET Year=%%C
)
:: Remove excess leading zeroes
SET Day=%Day:~-2%
SET Month=%Month:~-2%
:: Display the results
SET Day
SET Month
SET Year
:: Convert to Julian date
CALL :JDate %Year% %Month% %Day%
:: Display the result
SET JDate
:: Subtract 5 weeks
SET /A JPast = JDate - 5 * 7
:: Display the result
SET JPast
:: Convert back to "normal" date again
CALL :GDate %JPast%
:: Display the result
SET GDate
GOTO:EOF

Append the two subroutines to the code, and run it:

Day=25 
Month=04 
Year=2024 
JDate=2460427 
JPast=2460392 
GDate=2024 03 22

Check it, I couldn't catch it making any mistakes so far.

This is what I learned from Ron Bakowski.
Now I want to show you another neat trick that I learned from Paul Ruggieri — I added the weekday array myself:

:: Delayed variable expansion is required to read the array
SETLOCAL ENABLEDELAYEDEXPANSION
	•
	•
SET _Weekday.0=Monday
SET _Weekday.1=Tuesday
SET _Weekday.2=Wednesday
SET _Weekday.3=Thursday
SET _Weekday.4=Friday
SET _Weekday.5=Saturday
SET _Weekday.6=Sunday

:: Note: variable JDate must already have been set to a Julian Date

::Calculating day number [Monday = 0 ... sunday = 6]
SET /A WD = %JDate% %% 7
:: Display the result
SET Weekday=!_Weekday.%WD%!
	•
	•
ENDLOCAL

Sometimes we need to know how many days passed between two dates.
With Julian dates, that is a piece of cake: subtract the first date from the last one, and you have the number of days.

I was born on August 1, 1958, which is Julian date 2436418.
Today's Julian date is 2460427.
The difference, 24009, is my age in days (about 24009 / 365.25 = 65.73 years).

Would you ever have thought this would be possible in batch files?

Thanks again, Ron and Paul