[Lazarus] Lazarus Digest, Vol 70, Issue 35

Lex Edmonds ledmonds99 at gmail.com
Wed Nov 13 12:24:01 CET 2013

This is my first post on this mailing list, so please bear with me.
I used to do date calculations like this in the '80s when I was writing
Real Estate software.

On Tue, 12 Nov 2013 20:33:00, waldo kitty wrote:

> [...]
> the real question is this:
>    what do we count? the starting day, the ending day, both days or
> neither day?

There are two types of days-between-dates calculations: Inclusive and

Generally, the starting day should be considered the first day of the

With an Inclusive calculation, the ending day is counted.
With an Exclusive calculation, the ending day is not counted -- it is
considered to be the first day of the following period.

In Real Estate for example, date calculations are Inclusive.
So a 1-week rent payment that starts on Monday 1 January 2007 ends on
Sunday 7 January 2007.
Therefore an Inclusive DateDiff calculation between 1/01/2007 and 7/01/2007
would return 7 days.
Whereas an Exclusive calculation would return 6 days.

Similarly, when calculating a person's age.
A baby born on 1/01/2007 would be 7 days old on 7/01/2007.

As I recall, Exclusive dates seem to be more applicable to financial
For example, an Exclusive DateDiff calculation between 1/01/2007 and
1/04/2007 would return 3 months (not 3 months and 1 day).

then we have another question:
>    is dayM in monthY a one month difference to dayM in monthX and
>    monthZ? monthX, monthY, monthZ are any three consecutive months
>    in linear order within one year or crossing two years
> once the rules are chosen, then we have something to work with...

This is a topic that has some tricky boundary conditions.

Generally the rule is easy for days 1-27 in every month:
The Inclusive DateDiff between day M in month 1 and Day M-1 in month 2
returns 1 month.
e.g. between 28/01/2007 and 27/02/2007.

However the rules get muddy with days 28-31. i.e. the "last few" days of
every month.

For example, an Inclusive DateDiff calculation between 29/01/2007 and
28/02/2007 should return 1 month.
In my opinion, an Inclusive calculation
between 30/01/2007 and 28/02/2007 should also return 1 month.
And so should an Inclusive calculation between 31/01/2007 and 28/02/2007.

In fact, we used to call these "calendar months" to differentiate them from
"other" months that might always be defined as having perhaps 30 days.

So as you see, there is no real hard answer to your question. It probably
depend on what you are trying to achieve.

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