Advanced search page uses the DOB_FORMAT_STRING for displaying the correct format for the user. Hence the DOB_FORMAT_STRING has to be localized.

The bug is that the same DOB_FORMAT STRING is also used for checking the format and the zen_checkdate function, which does the checking, assumes that DOB_FORMAT_STRING is in the English language (e.g. it looks for dd and mm). Each and every check against non-english DOB_FORMAT_STRING is doomed to fail.

This bug is encountered at least with the Finnish language package.

I corrected the bug by defining separate format strings (in two different languages) for displaying and checking the date format. The English language version is used for checking with zen_checkdate_function in includes/modules/advanced_search_result/header.php

My zen cart version is v1.3.9e.