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Old 02-10-2011, 06:41 PM   #1
Registered: Jan 2006
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How to print out "n" days ago's year, month and day?

Interesting uh?
For example, 2 days ago's date is 2011 Feb 8th.
This is not a homework...

Old 02-10-2011, 07:51 PM   #2
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date -d '2 days ago'
Kevin Barry
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Old 02-10-2011, 08:22 PM   #3
Nominal Animal
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I was just going to write what ta0kira wrote. The GNU date command is awesome. Anyway, in C, you can do
#include <stdio.h>
#include <time.h>

int main(void)
    time_t     now, then;
    struct tm *split;

    /* Get the current time */
    now = time(NULL);

    /* Convert the time to broken-down format */
    split = localtime(&now);

    /* Substract two days -- it is okay to go out of valid range */
    split->tm_mday -= 2;

    /* Renormalize the broken-down format, and return the time then. */
    then = mktime(split);
    printf("Two days ago was %04d-%02d-%02d, or %s\n",
           split->tm_year + 1900, split->tm_mon + 1, split->tm_mday,

    return 0;
This uses the local time as a basis. If you need UTC, use gmtime instead of locatime.

The call to mktime both normalizes the split date, and returns the resulting time in the base time_t format, seconds since epoch. See the ctime, localtime, and mktime man page for details. This is in the POSIX standard, and should work as-is on all sane operating systems.

It shouldn't be too difficult to add parameter parsing to the above, to get exactly what you need.
Nominal Animal

Last edited by Nominal Animal; 03-21-2011 at 09:33 AM.
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Old 02-10-2011, 09:33 PM   #4
Registered: Apr 2010
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$ ruby -r'date' -e 'print ( - 48/24).strftime("%Y/%m/%d")'


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