DATEVALUE is used to convert a string that is formatted as a date into the date number value.  However, the actual date field in Forms doesn't store the date as a string, it stores it as a date, which naturally converts to a datevalue when copied from a date field into a number field.

You need to drop the DATEVALUE() part of your formula, and just keep the inner part of the formula, like this:    =INDEX(Measurements.DateOfMeasure,ROW())

When copying from a date field into a number field, it should look like this:

If for some reason your date was stored as a string (like in a Single Line field), then you'd need the DATEVALUE() part of your formula.