The EOMONTH function is an easy alternative to using the DATE function. With the EOMONTH function, you can get the last date of any future or past month. All you need is two arguments: a start date and the number of months in the future or past. For example, this formula returns the last day of April, 2015:
=EOMONTH("1/1/2015", 3)

Specifying a negative number of months returns a date in the past. So, for example, the following formula returns the last day of October, 2015:
=EOMONTH("1/1/2015", -3)

Specifying a zero as the number of months returns the last day of the month in which the given date falls. The following formula returns the last day of January, 2015:
=EOMONTH("1/1/2015", 0)