To validate the input of all date/time fields, you can use the following in a validation script (System Definition > Validation Scripts).

Because the date/time format is hard-coded in this script, it must match your instance's date/time format. If your instance's date/time format changes, you must update your validation script.

Set the validation script's type to "glide_date_time". Then, with this validation script, if a user enters an incorrect format in a date/time field, they will receive an error message.
function validate (value){ 
 if (!value) {
    return true;
 }

 return (getDateFromFormat(value, 'yyyy-MM-dd HH:mm:ss') != 0);
}

For a comprehensive use case, see Restricting record access.