Q:

Calculate Total Number Of Days Per Specific Month

Cell A1 shows the Start Date, e.g. 17/09/15
Cell B1 will need to reflect the End Date.End Date need to be based on 3 months of Start Date. 
I can't just use 90 days as standard days to add to the Start Date as different months have different no of days. Sept has 30 days, Oct 31 days and Nov 30 days.
Hence total number of days will be 91 days.
Is there any way excel can auto calculate the number of days in that specific month and add 3 months of days to Start Date? 

A:

Split the date up into year/month/day and add 3 months:  
=DATE(YEAR(A1),MONTH(A1)+3,DAY(A1))

assuming A1 holds your start date.

