Here are some Android applications I wrote for my own use that you might find helpful.
Calendricalc is a calculator that lets you add and subtract dates, convert dates to different time zones,
convert latitude and longitude to/from HH:MM:SS and decimal formats, convert atronomical coordinates,
and show the dates for US, Jewish, and Christian holidays.
A calculator to determine how many trunk lines or ports are required to support a given level of traffic. You will
need to provide the call arrival rate and the average call time.