You can write:
date +'%F %T'
which will print something like:
2012-03-04 11:56:54
(But as zerkms says, it's probably better to just use NOW() within MySQL.)
NOW()
Equivalent of @ruakh 's answer, in expanded form:
date +'%Y-%m-%d %H:%M:%S'
2014-04-14 11:33:48
Useful if some of the date stamp separator characters aren't valid for one use or another.