Unix Time (mtime)?

I have a command line that finds files in a folder and deletes the ones that are a over a day old. I can't seem to find a way/the write code to change the 1 (full) day into a half day (12) hours (f -mtime +1 -exec rm -rf {} ) = older than a day. Do you need to convert into milliseconds and then figure out what a half day is in milliseconds?