# [Lazarus] DateDif function needed

Hans-Peter Diettrich DrDiettrich1 at aol.com
Sun Nov 10 06:53:10 CET 2013

```waldo kitty schrieb:

>>    DecodeDate(Date2 - Date1,Y,M,D);
>>    // you have them in Y, M and D, respectively

> yeah, that doesn't work...
>
> Y = 1900
> M = 1
> D = 10
>
> Y should be 1 in this case...

I'm not sure, IMO also M and D should be decremented by 1.

> ideally, real numbers would be used so that fractions of years, months
> and days can be determined ;)

Of course real numbers are used. Problem is the time *base* (calendar
origin), that (here) translates zero into year 1900, and IMO also
offsets the month and day by 1. Delphi may yield a different year, AFAIR
its calendar is based on 1899-12-30 12:00.

The day *difference* is the integral part of real difference, i.e.
trunc(Date2-Date1). But for the elimination of all offsets I'd suggest
to determine all base values, and subtract these from the individual
values. Something like
DecodeDate(0.0, Y0,M0,D0);
DecodeDate(Date2-Date1,Y,M,D);
dec(Y,Y0); dec(M,M0); dec(D,D0);

But this still wouldn't be correct, because then the leap years
(february length) of 1900 ff. are taken into account, not those of the
start year. Better should be:
DecodeDate(Date1, Y0,M0,D0);
DecodeDate(Date2,Y,M,D);
dec(Y,Y0); dec(M,M0); dec(D,D0);
with the ugly effect that now M and D can become negative and have to be

So the general question arises: do you want a date difference expressed
in uniform years (of 360 days) and months (of 30 days), or in variable
calendar months and years?

IMO a dedicated Age function should be added, that determines e.g. the
age of a person properly. This function must return exactly zero months
and days for dates of the same month and day (birthday), regardless of
the years. Looks easy to accomplish, but is not really :-(

DoDi

```