The function signature in the validation error is:

addToDate (ClipboardPage, int, ClipboardPage, ClipboardPage, ClipboardPage)

The validation error you see is correctly indicating there is no function with the signature as it is provided in your case.

The actual function signature for addToDate in DateTime the should be "addToDate (String, String, String, String, String)" to match the existing function.

Here is the Help that you can see when you hover over the function in exporession builder that explains this:

Signature:

String addToDate (String theDate,String days,String hours,String minutes,String seconds)

Library : DateTime

Description :

Adds the given days, hours, minutes, and seconds to the DateTime passed in (or the current DateTime if no DateTime is passed) and returns the resulting DateTime as a String.

Usage :

This function will add days, hours, minutes, seconds to the given DateTime and will return a DateTime string.

Parameters:

theDate: base date to start with.

days: days to add to base date.

hours: hours to add to base date.

minutes: minutes to add to base date.

seconds: seconds to add to base date.

 Example:

addToDate('20090109T162504.370 GMT', '1', '1', '1', '1') = '20090110T172605.370 GMT'

To fix this validation error, try quoting the function parameters once added to the expression builder.

@DateTime.addToDate("theDate", "days", "hours", "minutes", "seconds")

e.g.

@DateTime.addToDate("20090109T162504.370 GMT", "1", "1", "1", "1")  which would give the result '20090110T172605.370 GMT'