Topic: Validation of input of a format of date

People kind, help please, I can not understand as generally it to write
To create the trigger checking correctness of adding of date in the table tv_series. If a wrong format to change it on true and to notify the user.
I do not understand how generally to work with a format,  here and all does not reach me. Help please .
The only thing that I understood so it that it is necessary to use before insert and to use to_char for date but as it should look at all I represent