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 **2457141** (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 **2457143**, 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 **2457143**, returns **2015 04 30**.

Not bad at all...

How about some date math? Let's try and find the date of 6 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 6 weeks SET /A JPast = JDate - 6 * 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=28 Month=04 Year=2015 JDate=2457143 JPast=2457101 GDate=2015 03 19

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 **2457143**.

The difference, **20725**, is my age in days (about 20725 / 365.25 = 56.74 years).

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

Thanks again, Ron and Paul