History of an individual file

Text mode

git log -p <filename>

Graphically

gitk <filename>

Find revision by date

This is kind of obtuse, and I really recommend git log or gitk . If you insist:

git rev-list -n 1 --before="2009-07-27 13:37" master

This finds one revision (-n 1) before a certain date. You can use this with git checkout (see below).

Check out old version of a file

Find the old revision by using git log (see above).

git checkout <revision> <file>

Reverting to the current version (HEAD):

git checkout HEAD <file>