Understanding Date Arithmetic

Most applications store dates as an integer and time as a decimal value. As a result, you can use simple calculations to create date- and time-specific values. For instance, to learn the number of days between two dates, you could use the simple formula
enddate - startdate
In a similar manner, you could return a date in the future through adding a value to a date in the form
startdate + days
Since one of your formula references contains a date, you may must format the cell containing the above formula as a Number cell to display the actual value.