The list below is a few simple functions which I've come across for generating various dates based on the current date/time (using the Now() function).

Last day of the current year,
=DateSerial(Year(Now()), 12, 31)

Last day of previous month
=DateAdd(""d"", -1, DateSerial(Year(Now()), Month(Now()), 1))

First date of previous month
=DateAdd(""m"", -1, DateSerial(Year(Now()), Month(Now()), 1))

First day of last year
=DateSerial(Year(Now())-1, 1, 1)

Last day of last year
=DateSerial(Year(Now())-1, 12, 31)

Last day of this year
=DateSerial(Year(Now()), 12, 31)

First day of this year
=DateSerial(Year(Now()), 1, 1)