Yes, IF is a function in Excel. It has to be entered while in "formula" mode (pressing the Function Key F2 on your keyboard). For example, say you want to know how many times "F" appears on all the sheets within the workbook. You would type:
=IF(ISBLANK("Sheet1"),0,TRIM(F))
into cell G3 and copy and paste that formula onto all cells following incrementally higher numbers. The result would look like this:
=IF(ISBLANK("Sheet1"),0,TRIM(FI))

The solution should be relatively straightforward in Excel (though it will take a little more time), and can be done by combining two nested IFs:
1) IF function with condition: if date field equals 0/01/1970 then result="never"
2) IF function with range of cells: if date field is not equal to 0/01/1970 then result=average(B4, B5, D4). With some minor modifications or adjustments for specific needs, one instance of a true nested IF should suffice in most cases.
Hopefully this resource will help
https://www.exceltrick.com/formulas_macros/excel-if-statement/