On May 22, 8:50 pm, clintonb <cba...@centurytel.netwrote:

On Apr 25, 5:42 pm, "Victor Bazarov" <v.Abaza...@comAcast.netwrote:

clintonb wrote:
I'm looking for a way to calculate the number of days between two

dates using standard C++ functions.

Would it be as simple as just using the difftime() function and then

dividing that result by the number of seconds in a day?

Sure, you could do that. Have you already tried? If not, why not?

If yes, why do you ask? Did it work?

I tried it. It seems to work.

Which, of course, doesn't mean much. Testing to find out

whether something is guaranteed to work is useless; it will only

tell you whether it worked with the exact input values you used,

on the implementation you tested. (This doesn't mean that you

shouldn't test your application, or unit test your individual

components. Only that you shouldn't count exclusively on

testing. And above all, that you shouldn't count at all on

testing to determine whether something is guaranteed by the

standard, on all implementations.)

The problem is that difftime returns the number of seconds

between the two *times*. Nothing about dates. So while the

number of days between May 23, 2007 and May 22, 2007 is 1,

that's not what you'll get if you simply do a difftime between

1:00 May 23, 2007 and 23:00 May 22, 2007. And any rounding

algorithm which will return 1 then will probably return 2 when

you compare 23:00 May 23, 2007 and 1:00 May 22, 2007. In order

to get correct results, you probably have to force the time_t to

a common time, by breaking them out into a tm struct, forcing

the tm_hour, tm_min and tm_sec to a "standard" value, then using

mktime to get a new time_t, and doing difftime on those. (Note

too that some systems store time_t in UTC, so local time zones,

summer time, etc., may have an influence on the results.)

I was actually trying to port some C# .NET code we had in one of our

applications to our C++ applications. .NET's datetime class has a

function for determining the number of days between two dates, so I

needed my C++ code to produce the same results. But I was wondering

if I had to handle stuff like daylight savings time, leap year, etc.

myself or whether the difftime() function took all that stuff into

account.

difftime gives the number of seconds between two times. There

are no leap years, daylight savings time, etc. in the number of

seconds. There are leap seconds, however (although some systems

choose to ignore them).

If you're only concerned about local time, calling localtime on

the two values, forcing the time to noon, then using mktime to

convert back to time_t, difftime on those, and dividing by the

number of seconds in a day, and rounding the results to nearest

should be adequate. The results of difftime could be off about

an hour, if there was a shift between summer time and standard

time in the interval; the error should never be enough that

rounding to an integral number of days doesn't give the correct

results, however.

--

James Kanze (GABI Software) email:ja*********@gmail.com

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