Time calculator

I wish there were a calculator that handled time durations. Some expressions to evaluate:

today

today + 200 days

4 years / 37

now + (6 day)/15

17:00 - 3:15

now - 4y6mo3d7h12m18s

17june-12april

(midnight june 19th)-(7pm on 12april) in minutes

Perhaps this can be an improvement on Google Calculator.