the less-than-or-equals operator ‘<=’ returns either TRUE or FALSE and stores that in the column
which is used to set the Check-Box accordingly

the formula for showing the difference between the DUE DATE and the COMPLETED DATE is a simple subtraction…

[COMPLETED DATE] - [DUE DATE]

because dates are stored as decimal numbers, and the integer part is the number of days since the base-date (which i think is 01 Jan 1970) - but we only want the integer part of the result, so we can use Floor(1) to drop the fractional part (the 1 implies to round down to the nearest integer).

BTW: the fractional part of the date value encodes the hours, minutes and seconds, so we drop those

But we also need to allow for the case where the COMPLETED DATE is not yet entered, so we use an If() function for that to return a blank result in that case. In the If() function, an empty cell is FALSE, otherwise it is TRUE

So the final formula might be something like this…