Convert date (year month day) to day count

问题

I would like a function (using Gregorian calendar rules) given 3⍴⎕TS would return 1+ the number of days since 1 1 1. EG if the function were named X:

      X¨ (1 1 1) (1 12 31) (2 12 31) (3 12 31) (4 12 31)
1 365 730 1095 1461

I need this so I can subtract two dates to find the number of days between them.

回答1:

Please see the following example - function X assumes index origin 1 as follows:

r←X b  
r←(0 31 59 90 120 151 181 212 243 273 304 334)[b[2]]    
r←r+b[3]+(365×b[1]-1)+-/⌊(b[1]-3>b[2])÷4 100 400

回答2:

In Dyalog APL, you can make use of the interpreter's built-in DateToIDN and IDNToDate methods to convert to and from an International Day Number, and thus allow interger math.

You can conveniently package them as follows:

ToIDN ← {⊢ 2 ⎕NQ # 'DateToIDN' , ⍵}
ToDate ← {3 ⍴ 2 ⎕NQ # 'IDNToDate' , ⍵}

Try it online!

The ⊢ is to counter ⎕NQ being shy, and the 3⍴ is to chop off the weekday number which is appended as a 4^th element.

You can additionally define an operator which allows you to operate on dates seamlessly:

Datewise ← {ToDate (ToIDN ⍺) ⍺⍺ ⍵}

Try it online!

Finally, if you are not using Dyalog APL, or if you are interested in the actual formulas to determine the IDN, have a look at the days and date functions from the dfns workspace.

来源：https://stackoverflow.com/questions/45406404/convert-date-year-month-day-to-day-count