You have 3 options:
... | eval Date = coalesce(Date, yourAlternateFieldHere, "Your Hard-Coded Default Value Here")
... | fillnull value="Your Hard-Coded Default Value Here" Date
... | filldown Date
I leave it as an exercise for the reader to figure our (or @DalJeanis to explain) the differences.
You need to populate the Date field.
Identify, on the records with no Date, what is a good candidate for substitution, then do something like
| eval Date = coalesce(Date,mySubstituteField)