You need to apply a format to your data.
format sevendays_after DATETIME17.;
Or DTMONYY.
or similar.
You should use INTNX, in any event.
sevendays_after = intnx('DTDAY',creation_date,7);
That way you don't have to do the math.
If you just want the date, and would like to discard the time information entirely, you can use datepart
to do that (although that should only be done if the time part is useless, such as a date-stamp that is forced into datetime by SQL Server).