I have added a new column to an out-of-the-box tasks list called CompletedDate and formatted it as date time and added some data to my tasks list. I made sure to add at least one date where the completedDate is later than the Due Date. I customized the default view to only show the relevant columns.(not shown)

In the ribbon I click Create Column. For this example I will use a list column.

I call my column DaysEarly and choose calculated column as the data type. This opens up the Formula section.

The formula is part typing and part clicking the column options on the right hand side. I have not had good luck typing column names. I type the formulas but when it’s time to insert the column name I always double-click the column name from the Insert Column box on the righ to place it into my formula. This particular formula prefixes the date difference calculation formula just in case I am late and puts in the string “LATE” if I am, otherwise it returns a number of days I am early.

The formula in its entirety is:

IF(ISERROR(DATEDIF(completedDate,[Due Date],”d”)),”LATE”,DATEDIF(completedDate,[Due Date],”d”))

The first part of the formula checks to see if my calculation will return an error and if it does, it will return the value of LATE as the column value, otherwise it will perform the calculation and return the number of days (“d”) between the two dates where the completedDate is earlier than the Due Date as a number.

It all looks good. I can see the days early appearing and the one blog I entered as being late has the value of the DaysEarly column set as LATE.