The Signtool Timestamp Files action allows you to apply a timestamp to files that have already been signed.
Specify the files that you want to apply the timestamp to (each entry on a new line). The files specified must have already been digitally signed otherwise the action will fail.
Specify the Timestamp Service URL that will be used obtain the timestamp.
You can also use the Verbose Output option with this action to obtain extended output from the Signtool executable.
For more information on Signtool see MSDN.