# Calculate the date after subtracting nmbr of days form a date

Discussion in 'C Programming' started by Laery, Feb 24, 2005.

1. ### LaeryGuest

Hi,

I'm currently adding a new module to an old borland C3.1 application
(dos).
And I need to calculate a date by subtracting the number of days from
a given date.

I know I could use an array of days in the months and go back by
leaping when 1 is reached. (keeping the 29the of feb and january/year
in mind).
But is there no function in Borland C3.1 which will do this for me?

I couldn't find one in the help?

Regards
Laery

Laery, Feb 24, 2005

2. ### JohnGuest

(Laery) wrote in
news::

> I'm currently adding a new module to an old borland C3.1 application
> (dos).
> And I need to calculate a date by subtracting the number of days from
> a given date.

[...]

> I couldn't find one in the help?

Look-up difftime and localtime... They're functions that have been in the
Borland libraries at least since v1.5 and should help you do what you're
trying to do.

John

John, Feb 24, 2005

3. ### infobahnGuest

Laery wrote:
>
> Hi,
>
> I'm currently adding a new module to an old borland C3.1 application
> (dos).
> And I need to calculate a date by subtracting the number of days from
> a given date.
>

Load the date into a struct tm, taking care to ensure that:

(a) all fields you don't set are cleared to 0, eg with:
struct tm date = (0); /* for now, just trust me on this */
(b) the year field contains the full year less 1900 (so, for 2005,
it would be set to 105).
(c) the month field is in the range 0 to 11.
(d) all other relevant fields are set correctly.

Now subtract the number of days you want, and then pass the
struct's address to mktime(), catching the result in a time_t
object.

If the result is not (time_t)-1 (which would indicate failure to
convert the date as you require), it can be passed to localtime()
or gmtime(), which both return pointers to struct tm from which
you can extract the date information you require.

infobahn, Feb 24, 2005
4. ### Richard BosGuest

John <> wrote:

> (Laery) wrote in
> news::
>
> > I'm currently adding a new module to an old borland C3.1 application (dos).
> > And I need to calculate a date by subtracting the number of days from
> > a given date.

>
> > I couldn't find one in the help?

>
> Look-up difftime and localtime... They're functions that have been in the
> Borland libraries at least since v1.5 and should help you do what you're
> trying to do.

No, they won't. difftime() gives the difference, in seconds, between two
time_t's; it doesn't allow you to change an existing time_t. localtime()
converts from time_t to struct tm, but doesn't do any other
computations.
Using mktime() is the right solution. If BC3.1 doesn't have it yet
(i.e., if it's pre-ISO), you _may_ be able to get away with subtracting
days*24*3600 from a time_t, but do note that this relies on time_t being
a straight number of seconds since the epoch, which it isn't required to
be. Doing so would make your code unportable, but if it's already
Borland-specific, that may not be a problem. Adding a comment explaining
the hack would be a good idea, even so.

Richard

Richard Bos, Feb 24, 2005
5. ### Stan MilamGuest

Laery wrote:
> Hi,
>
> I'm currently adding a new module to an old borland C3.1 application
> (dos).
> And I need to calculate a date by subtracting the number of days from
> a given date.
>
> I know I could use an array of days in the months and go back by
> leaping when 1 is reached. (keeping the 29the of feb and january/year
> in mind).
> But is there no function in Borland C3.1 which will do this for me?
>
> I couldn't find one in the help?
>
> Regards
> Laery

I have a really robust date library which was published in the C User's
Journal a few years ago. Drop me a line and I will email it to you
(damn, I've got to get that web page up!). I also used it to write a
Unix shell utility called ADU (a date utility) that allows you to do
date math in the Unix shell. If there were a decent shell for Windows
you could use it there too.

Stan Milam, Feb 24, 2005
6. ### Jack KleinGuest

On Thu, 24 Feb 2005 11:04:15 +0000 (UTC), infobahn
<> wrote in comp.lang.c:

> Laery wrote:
> >
> > Hi,
> >
> > I'm currently adding a new module to an old borland C3.1 application
> > (dos).
> > And I need to calculate a date by subtracting the number of days from
> > a given date.
> >

>
> Load the date into a struct tm, taking care to ensure that:
>
> (a) all fields you don't set are cleared to 0, eg with:
> struct tm date = (0); /* for now, just trust me on this */

ITYM struct tm date = { 0 };

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html

Jack Klein, Feb 25, 2005
7. ### Randy HowardGuest

In article <>,
says...
> Laery wrote:
> >
> > Hi,
> >
> > I'm currently adding a new module to an old borland C3.1 application
> > (dos).
> > And I need to calculate a date by subtracting the number of days from
> > a given date.
> >

>
> Load the date into a struct tm, taking care to ensure that:
>
> (a) all fields you don't set are cleared to 0, eg with:
> struct tm date = (0); /* for now, just trust me on this */

It would be more trustworthy with braces instead of parens.

--
Randy Howard (2reply remove FOOBAR)
"Making it hard to do stupid things often makes it hard
to do smart ones too." -- Andrew Koenig

Randy Howard, Feb 25, 2005
8. ### infobahnGuest

Jack Klein wrote:
>
> On Thu, 24 Feb 2005 11:04:15 +0000 (UTC), infobahn
> <> wrote in comp.lang.c:
>
> > (a) all fields you don't set are cleared to 0, eg with:
> > struct tm date = (0); /* for now, just trust me on this */

>
> ITYM struct tm date = { 0 };

I do indeed. I was clearly tempting fate a little too much with that
comment.

infobahn, Feb 25, 2005
9. ### infobahnGuest

Randy Howard wrote:
>
> In article <>,
> says...
> > Laery wrote:
> > >
> > > Hi,
> > >
> > > I'm currently adding a new module to an old borland C3.1 application
> > > (dos).
> > > And I need to calculate a date by subtracting the number of days from
> > > a given date.
> > >

> >
> > Load the date into a struct tm, taking care to ensure that:
> >
> > (a) all fields you don't set are cleared to 0, eg with:
> > struct tm date = (0); /* for now, just trust me on this */

>
> It would be more trustworthy with braces instead of parens.

<sigh>
Some days, it just doesn't pay to fire up your newsreader. :-\$
</sigh>

infobahn, Feb 25, 2005
10. ### dragoncoderGuest

Hello Stan

If you could mail me a copy of the date library you are talking about,
that will be very kind of you. Just mail it to me at

ptiwaryATmahindrabtDOTcom

Thanks in advance.

dragoncoder, Feb 25, 2005
11. ### Randy HowardGuest

In article <>,
says...
> <sigh>
> Some days, it just doesn't pay to fire up your newsreader. :-\$
> </sigh>

Unfortunately, I know exactly what you mean.

--
Randy Howard (2reply remove FOOBAR)
"Making it hard to do stupid things often makes it hard
to do smart ones too." -- Andrew Koenig

Randy Howard, Feb 25, 2005
12. ### LaeryGuest

Hi,

Thanks for all the responses.

Regards
Laery
(Laery) wrote in message news:<>...
> Hi,
>
> I'm currently adding a new module to an old borland C3.1 application
> (dos).
> And I need to calculate a date by subtracting the number of days from
> a given date.
>
> I know I could use an array of days in the months and go back by
> leaping when 1 is reached. (keeping the 29the of feb and january/year
> in mind).
> But is there no function in Borland C3.1 which will do this for me?
>
> I couldn't find one in the help?
>
> Regards
> Laery

Laery, Feb 25, 2005

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