Avoiding #NAME? Error/ Calculating date difference

I am creating a share point list that tracks our plan progress through the year. I have 2 date columns - [planned start date] and [actual start date].

I have the current formula in place:

=IF(ISBLANK([Activity Start Date]),"0",DATEDIF([Planned Start Date],[Activity Start Date],"YM"))

However, When the[actual start] is BEFORE the planned date, it gives me the #NUM? error rather than a negative value.

I have found previously the suggestion to use a formula:

=IF(ISERROR(DATEDIF(TODAY(),[Due date],"d")),"0",DATEDIF(TODAY(),[Due date],"d"))

I adapted it to fit my list:

=IF(ISBLANK([Activity Start Date]),"0",IF(ISERROR(DATEDIF([Planned Start Date],[Activity Start Date],"YM")),"0",DATEDIF(([Planned Start Date],[Activity Start Date],"YM"))

However, received a syntax error.