Aha - date arithmetic.
You could set up an array with 28/29/30/31 days lookups for the months and leapyear calculations.

I once wrote a C program called timecalc. Here it is, if it's of any use:

Code:

/*
 * Name:
 *	timecalc.c
 *
 * Purpose:
 *	Time calculations.
 #
 * Usage:
 *	timecalc [+|-]n [seconds|minutes|hours|days|weeks]
 *
 * History:
 *	050713	GZB	Original.
 *
 */

static char *gsVersion="@(#)timecalc.c GZB 050713";

#include <time.h>
#include <stdlib.h>
#include <sys/time.h>

int gbDebug=0;
int gbVerbose=0;
double gfTimeDiff=0;	/* What to change the time by. */
int giTimeDiff=0;	/* What to change the time by. */
int giTimeUnits=1;	/* Time units scaled to seconds. */
char gsDateSep[10]="/";	/* Date separator. */
char gsTimeSep[10]=":";	/* Time separator. */
char gsDtoTSep[10]=" ";	/* Separator between date & time. */


Usage()
{
	printf("usage: timecalc [+|-]n [seconds|minutes|hours|days|weeks]\n");
	exit(1);
}


CommandArgs(argc,argv)
	int argc;
	char **argv;
{
	int i;

	if (gbDebug) printf("CommandArgs()\n");

	for ( i=1; i<argc; i++ )
	{
		if (gbDebug) printf("%d/%d\n", i, argc);

		switch (*argv[i])
		{
			case '+': gfTimeDiff=atof(argv[i]+1); break;
			case '-': gfTimeDiff=-atof(argv[i]+1); break;
			case 's': giTimeUnits=1; break;
			case 'm': giTimeUnits=60; break;
			case 'h': giTimeUnits=60*60; break;
			case 'd': giTimeUnits=60*60*24; break;
			case 'w': giTimeUnits=60*60*24*7; break;
			case 'D': gbDebug=1; break;
			case 'V': gbVerbose=1; break;
			/* case 'T': *gsDateSep='\0'; *gsTimeSep='\0'; break; */
			case 'T': strcpy(gsDateSep,""); strcpy(gsTimeSep,""); break;
			default: Usage(); break;
		}
	}

	/*
	giTimeDiff *= giTimeUnits;
	if (gbDebug) printf( "%d %d\n", giTimeDiff, giTimeUnits);
	*/

	giTimeDiff = (int)(gfTimeDiff * (double)giTimeUnits);
	if (gbDebug) printf( "%d %f %d\n", giTimeDiff, gfTimeDiff, giTimeUnits);
}


PrintTime(tp)
	struct tm *tp;
{
	printf("%04d%s%02d%s%02d%s%02d%s%02d%s%02d",
		tp->tm_year+1900,
		gsDateSep,
		tp->tm_mon+1,
		gsDateSep,
		tp->tm_mday,
		gsDtoTSep,
		tp->tm_hour,
		gsTimeSep,
		tp->tm_min,
		gsTimeSep,
		tp->tm_sec
		);
}


main(argc,argv)
	int argc;
	char **argv;
{
	time_t	t;
	time_t	tnew;
	struct tm *tp;

	t = (time_t)time((time_t *)0);
	if (gbDebug) printf("Seconds since epoch = %d\n", t);

	CommandArgs(argc,argv);

	tnew = t + giTimeDiff;
	if (gbDebug) printf("Seconds adjusted by %d = %d\n", giTimeDiff, tnew);

	if (gbVerbose)
	{
		tp = localtime(&t);
		PrintTime(tp);
		printf("\n");
		/* printf(" -> "); */
	}

	tp = localtime(&tnew);
	PrintTime(tp);
	printf("\n");
}

All usual disclaimers, non-warranties, and so-on apply!

Alternatively, you could try and make some clever use of the "cal" (calendar) program.