Use a time unit class, but hold a concrete time type (http://rel4tion.org/projects/time-interval/)
See the .cabal file for more info and link to project website the version control.
The official download location is Hackage:
This library is free software, and is committed to software freedom. It is released to the public domain using the CC0 Public Domain Dedication. For the boring "legal" details see the file 'COPYING'.
See the file 'INSTALL' for hints on installation. The file 'ChangeLog' explains how to see the history log of the changes done in the code. 'NEWS' provides a friendly overview of the changes for each release.