formula which tells me when something is due in days

welcome to the forum, sally. assuming licence due date in A2, to get the date 4 months later would be:
=EDATE(A2,4)
so if A2 is 29 Dec 2014, you would get 29 Apr 2015

to count how many days is left to that day from today would be:
=EDATE(A2,4)-TODAY()
you would then get 103 days if today is 16 Jan 2015.

if you have a case where due date + 4 months is before today's date, and you want to show a text like "expired", then:
=IF(EDATE(A2,4)-TODAY()<0,"Expired",EDATE(A2,4)-TODAY())

for a certain length of time, say <30 days, try this:
select the range you want to apply to (say from A2:B20)
go to Home -> Conditional Formatting -> New Rule -> Use a formula to determine which cells to format -> Format values where this formula is true:
=EDATE($A2,4)-TODAY()<30
format colour

ps: havent seen your file while i was replying