how to assign a string value to a date field

Hi everyone,
I wrote the following formula:
Basically, I want to say if the variable equals to null or that particular value, then assign the "waiting" to it otherwise show me the actual value. But, it seems it doe not work and I still see '12/30/1999' as the value. Any help on this, please

=If IsNull( [Task Closed Date Time]) Or   FormatDate([Task Closed Date Time];"mm/dd/yy") ="12/30/99"
Then "Wating" Else [Task Closed Date Time]