For me, NULL is definitely not «Unknown», since «Unknown» is a result of a logical operation.

Being in doubt i voted for NULL, but the question should have had more explanation.

Another useful statistics statement is «Time», it will show you the number of milliseconds for parsing, compiling and executing your statements:

Set Statistics TIME ON

exec your Sp

Set Statistics TIME OFF

A good practice comes from a well-known phrase:

«There's only two men I trust. One of them is me. The other's not you.» 🙂

I try to put constraints on every table...

I agree with Lynn,

a Datetime presents a much better/faster/smaller/easier solution, but if you somehow are required(obliged) to use the varchar, then use a contraint with a user defined function to...