Command-line Tools for fiddling with Dates and Times
http://hroptatyr.github.com/dateutils/

Dateutils are a bunch of tools that revolve around fiddling with dates and
times in the command line, with a strong focus on use cases that arise when
dealing with large amounts of financial data. Their target market is shell
scripts that need date calculations or calendar conversions, and as such they
are highly pipe-able and modeled after their well-known cousins (e.g. dtest vs.
test, or dgrep vs. grep).