Need help with 'If , Then' function to subtract Duration Values

I'm setting up a Construction Schedule with Variance calculations as the 'Near Term' baseline solution that's been quoted as coming for a few years now has not materialized yet.

Unfortunately after searching the forums I'm not seeing syntax examples that work for what I need to do. It would be helpful for me at least to see more synatx examples listed and I have found these in the forums primarily as responses to questions but still can't get this to work and now just need to finish this up quickly.

Below is a screen shot, I can calculate the duration between two columns as long as there are positive numbers using: =([Actual Duration]@row - [PLAN DURATION]@row) When there is a 0 value however there is the error #INVALID OPERATION that I'd like to get rid.

In the first row I tried the following If , Then but must be missing a space or character or something simple since some variation of this should work to avoid the #UNPARSEABLE error.

=IF [PLAN DURATION] > 0, ([Actual Duration]@row - [PLAN DURATION]@row)