Custom Column in Transformation, irrational errors

Power bi has been kicking my arse all morning by telling me that statements in the Custom Column diolog in the Transformation tab are syntatixcally correct (No errors found) yet after clicking ok, telling me stupid things like Duration.Day is not recognized.

Computed colum:

if [MySkills.TargetDate] is null
then
Duration.Day(DateTime.LocalNow() - DateTime.LocalNow())

else

Duration.Day(DateTime.LocalNow() - [MySkills.TargetDate])

Syntax is green lighted by Power bi, but then returns "Expression.Error: The name 'Duration.Day' wasn't recognized. Make sure it's spelled correctly."