C2 is where we put the code and C5 is the date. Where A2 is a cell containing the date. For example DATE2015 15 5 returns the serial number representing March 1 2016 January 5 2015 plus 15 months.
If you want to see the serial number in date format change to date.
If you add multiples of 365 to a given date then it will give an incorrect answer since there can be a leap year in between. The Excel DATE function creates a valid date from individual year month and day components. The serial number is the calculation of a number of days from the 01011900 to a given date. Step - the increment for each subsequent date in a sequence.