The DATE Function is one of Excel's built in formulas.
The function is often used to ensure that dates entered into other formulas are in the correct format so that they can be used in calculations.
The DATE function can also be used to convert a given date into the computer's serial date.
The date information that is used by the function is referred to as the function's arguments.
The DATE function has three arguments - the year, the month, and the day that is to be formatted by the function.
These arguments can be entered directly into the function or the cell references to the location of the data in the worksheet can be entered instead
Location in Excel
All functions in Excel can be found under the Formulas tab of the ribbon. The DATE function is located in the Date & Time Function Group
More information: Excel DATE Function Tutorial