|Dates and Times are stored within Excel as a number. The integer portion is the Date, the decimal portion is the Time.|
Since each is Day is equal to 1, simply adding the number of days to a Due Date will give you a new date.
With 1/1/2013 in A1, this formula will return 4/1/2013, since that is 90 days later:
See here for a good explanation of how Excel deals with Dates and TImes:
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