Beginner's tips please

How would I correctly represent represent the following:
a) emit invoice after which
b) receive payment WITHIN 60 days
or this other dependency
a) request document
b) receive response within 3 months

lastly another tip please

for my application I want to always have 52 weeks per year, 5 weeks per month (in other words when I express a period in years it needs to contain 52 weeks but when I express a period in months it should be 5 weeks.

Thank you