ctime & find

I am trying to figure out the syntax to use find to remove files older than 30 minutes. I know that this will work for files 1 day old, but cannot seem to trim the time down to 30 minutes.

find /path/to/file -ctime +1 -exec rm -f {} \;

You can "touch" a file with a specified datestamp using the "-t" option. Then use find with the "! -cnewer <I>file</I>" options.