Insert into what?

generated by localtime()

Generated where? In the same script that is inserting? Some other process? A file? A database?

and get the difference in seconds.

If the two time stamps have been generated within the same script that is inserting them, the difference in seconds (as mentioned in a previous reply) is the difference in the values returned by time at each moment when those time stamps were generated.

If it's a really small task, maybe you could show us the relevant code you have (or even just pseudo-code) so we know what you're really trying to do.