"Whenever we use any data type we should know it's default value"

I disagree (even ignoring the valid but slightly pedantic points about datatypes not having defaults).

We should never rely on default values in this way, it is prone to error, and hard to read the code.

If you want to assign a value to a date variable, then explicitly assign the value you want, don't give it a value of a different datatype and rely on implict conversion 🙂