I ran into this date and time constraint problem earlier this week, and haven't really found any good approach for an algorithm. Each idea I get grinds to a halt with something like what if this is a leap year? or what if this is run on the night when we change to/from DST
Input: A crontab expression (wikipedia on CRON format,Cron). For example:
0 */5 2,14 * * *
Meaning Every five minutes when the hour is 2 or 14
30 5 */2 1 * * */2
Meaning 5:30 past every even hour on the first of each month, every even year.
Output: The last time this expression was true. If run at 11:00 AM on november first, 2009, the output should be:
2009-11-01 02:55:00
for the first example and
2008-11-01 10:05:30
for the second.
Some notes:
There seems to be a couple of variations of the cron expression format: Some include seconds, some include year. The general problem should be about the same.
Feel free to apply sensible constraints; It's for example perfectly ok to don't handle years before 1970.
My current gut-feeling is that a depth-first search from year all the way down to second, backtracking when we run into invalid dates and times.
A brute-force approach could be to count backwards one day at a time, and evaluate the date part of the cron expression (there has only been about 15k days since 1970). When a valid day is found, do the same for the time part.
Answers doesn't have to contain code, I'm mostly after an algorithm outline like the ones above.