How to set git commit timestamp

Hello,

does somebody know a method to specify a timestamp for a 'git commit'?

I am writing a tool to convert from another SCM to git and need to replay all checkins. I know how to set the log message and the author, but there appears to be no command option for the timestamp (Mercurial for example, has a 'hg commit -d date <file>' syntax for this).

Any help is appreciated.

Regards

Guido