i want to make a link that is valid only for 24 hours, this is for a validation purpose, so my question is simple:

How do i make this link valid only for this time; i've a hint:

1. Get the epoch time.
2. Make a link using only this value: something.com/time/1359380374
3. When the user clics on the link, extract this value and compare.

I hear about Hash values? why? we cant get the time from the hash value (invert the process) so how this is done?