EditPad Lite has a nice feature (CTRL-E, CTRL-I) which inserts a time stamp e.g. \"2008-09-11 10:34:53\" into your code.
As an extension to @Swaroop C H's answer,
^R=strftime("%FT%T%z")
is a more compact form that will also print the time zone (actually the difference from UTC, in an ISO-8601-compliant form).
If you prefer to use an external tool for some reason,
:r !date --rfc-3339=s
will give you a full RFC-3339 compliant timestamp; use ns instead of s for Spock-like precision, and pipe through tr ' ' T to use a capital T instead of a space between date and time.
Also you might find it useful to know that
:source somefile.vim
will read in commands from somefile.vim: this way you could set up a custom set of mappings, etc., and then load it when you're using vim on that account.