Please look at
/admin/customers.php

Starting at line 138:
Code:
        if (ACCOUNT_DOB == 'true') {
          if (ENTRY_DOB_MIN_LENGTH >0) {
            if (checkdate(substr(zen_date_raw($customers_dob), 4, 2), substr(zen_date_raw($customers_dob), 6, 2), substr(zen_date_raw($customers_dob), 0, 4))) {
              $entry_date_of_birth_error = false;
            } else {
              $error = true;
              $entry_date_of_birth_error = true;
            }
          }
        } else {
            $customers_dob = '0001-01-01 00:00:00';
        }
should be:
Code:
        if (ACCOUNT_DOB == 'true') {
          if (ENTRY_DOB_MIN_LENGTH >0) {
            if (checkdate(substr(zen_date_raw($customers_dob), 4, 2), substr(zen_date_raw($customers_dob), 6, 2), substr(zen_date_raw($customers_dob), 0, 4))) {
              $entry_date_of_birth_error = false;
            } else {
              $error = true;
              $entry_date_of_birth_error = true;
            }
          } else {
            $customers_dob = '0001-01-01 00:00:00';
          }
        }
Please correct me if I'm wrong! But I found this when merging my own mods into customer.php.
-Chadderuski