Check if date is is greater than a specific date

Check if date is is greater than a specific date

I want to create a calculated field based on the date of another date field.  The field is called Project Start Date and what I would like to do is create another field called October 2021 (I will repeat this for each month) and if the Projects Start Date is greater than "10/01/2021" then get some calculated values.  Here is what I have so far but have been getting errors.

If (
  ${Deals.Estimated Completion Date} <  Datepart( Newdate( 2021 ,01 ,10 ,06 ,30 ,'AM' ) ),
  ${Deals.Total Man Hours},
  0.00
)

But I get this error: 
You used the wrong type of data in an argument. Check what you entered between the parentheses of your functions to make sure you put the right kind of data in the right place.

The formula return type is decimal as is ${Deals.Total Man Hours}