Thanks gilby, I eventually got it working.

For anyone else who wants to know here is what I done step by step to change the date format from mm/dd/yyyy to dd/mm/yyyy:

1. open file includes/languages/<YOUR_TEMPLATE>/<YOUR_LANGUAGE>.php

2. find this section around line 22:
@setlocale(LC_TIME, 'en_US.ISO_8859-1');
define('DATE_FORMAT_SHORT', '%m/%d/%Y'); // this is used for strftime()
define('DATE_FORMAT_LONG', '%A %d %B, %Y'); // this is used for strftime()
define('DATE_FORMAT', 'm/d/Y'); // this is used for date()
define('DATE_TIME_FORMAT', DATE_FORMAT_SHORT . ' %H:%M:%S');
and replace with:
@setlocale(LC_TIME, 'en_US.ISO_8859-1');
define('DATE_FORMAT_SHORT', '%d/%m/%Y'); // this is used for strftime()
define('DATE_FORMAT_LONG', '%A %d %B, %Y'); // this is used for strftime()
define('DATE_FORMAT', 'd/m/Y'); // this is used for date()
define('DATE_TIME_FORMAT', DATE_FORMAT_SHORT . ' %H:%M:%S');
3. in the same file find this section around line 29:
// Return date in raw format
// $date should be in format mm/dd/yyyy
// raw date is in format YYYYMMDD, or DDMMYYYY
if (!function_exists('zen_date_raw')) {
function zen_date_raw($date, $reverse = false) {
if ($reverse) {
return substr($date, 3, 2) . substr($date, 0, 2) . substr($date, 6, 4);
} else {
return substr($date, 6, 4) . substr($date, 0, 2) . substr($date, 3, 2);
}
}
}
and replace with:
// Return date in raw format
// $date should be in format mm/dd/yyyy
// raw date is in format YYYYMMDD, or DDMMYYYY
if (!function_exists('zen_date_raw')) {
function zen_date_raw($date, $reverse = false) {
if ($reverse) {
return substr($date, 0, 2) . substr($date, 3, 2) . substr($date, 6, 4);
} else {
return substr($date, 6, 4) . substr($date, 3, 2) . substr($date, 0, 2);
}
}
}
4. In the same file find this section around line 67:
// text for date of birth example
define('DOB_FORMAT_STRING', 'mm/dd/yyyy');
and replace with:
// text for date of birth example
define('DOB_FORMAT_STRING', 'dd/mm/yyyy');
5. In the same file find this section around line 200:
define('ENTRY_DATE_OF_BIRTH_ERROR', 'Is your birth date correct? Our system requires the date in this format: MM/DD/YYYY (eg 05/21/1970)');
define('ENTRY_DATE_OF_BIRTH_TEXT', '* (eg. 05/21/1970)');
and replace with:
define('ENTRY_DATE_OF_BIRTH_ERROR', 'Is your birth date correct? Our system requires the date in this format: DD/MM/YYYY (eg 21/05/1970)');
define('ENTRY_DATE_OF_BIRTH_TEXT', '* (eg. 21/05/1970)');
6. Open file admin/includes/languages/<YOUR_LANGUAGE>.php

7. Find this section around line 20:
setlocale(LC_TIME, 'en_US.ISO_8859-1');
define('DATE_FORMAT_SHORT', '%m/%d/%Y'); // this is used for strftime()
define('DATE_FORMAT_LONG', '%A %d %B, %Y'); // this is used for strftime()
define('DATE_FORMAT', 'm/d/Y'); // this is used for date()
define('PHP_DATE_TIME_FORMAT', 'm/d/Y H:i:s'); // this is used for date()
define('DATE_TIME_FORMAT', DATE_FORMAT_SHORT . ' %H:%M:%S');
define('DATE_FORMAT_SPIFFYCAL', 'MM/dd/yyyy'); //Use only 'dd', 'MM' and 'yyyy' here in any order
and replace with:
setlocale(LC_TIME, 'en_US.ISO_8859-1');
define('DATE_FORMAT_SHORT', '%d/%m/%Y'); // this is used for strftime()
define('DATE_FORMAT_LONG', '%A %d %B, %Y'); // this is used for strftime()
define('DATE_FORMAT', 'd/m/Y'); // this is used for date()
define('PHP_DATE_TIME_FORMAT', 'd/m/Y H:i:s'); // this is used for date()
define('DATE_TIME_FORMAT', DATE_FORMAT_SHORT . ' %H:%M:%S');
define('DATE_FORMAT_SPIFFYCAL', 'dd/MM/yyyy'); //Use only 'dd', 'MM' and 'yyyy' here in any order
8. In the same file find this section around line 29:
// Return date in raw format
// $date should be in format mm/dd/yyyy
// raw date is in format YYYYMMDD, or DDMMYYYY
function zen_date_raw($date, $reverse = false) {
if ($reverse) {
return substr($date, 3, 2) . substr($date, 0, 2) . substr($date, 6, 4);
} else {
return substr($date, 6, 4) . substr($date, 0, 2) . substr($date, 3, 2);
}
}
and replace with:
// Return date in raw format
// $date should be in format mm/dd/yyyy
// raw date is in format YYYYMMDD, or DDMMYYYY
function zen_date_raw($date, $reverse = false) {
if ($reverse) {
return substr($date, 0, 2) . substr($date, 3, 2) . substr($date, 6, 4);
} else {
return substr($date, 6, 4) . substr($date, 3, 2) . substr($date, 0, 2);
}
}
9. In the same file find this section around line 85:
// text for date of birth example
define('DOB_FORMAT_STRING', 'mm/dd/yyyy');
and replace with:
// text for date of birth example
define('DOB_FORMAT_STRING', 'dd/mm/yyyy');
10. In the same file find this section around line 204:
define('JS_DOB', '* The \'Date of Birth\' entry must be in the format: xx/xx/xxxx (month/date/year).\n');
and replace with:
define('JS_DOB', '* The \'Date of Birth\' entry must be in the format: xx/xx/xxxx (date/month/year).\n');
11. In the same file find this section around line 231:
define('ENTRY_DATE_OF_BIRTH_ERROR', '&nbsp;<span class="errorText">(eg. 05/21/1970)</span>');
and replace with:
define('ENTRY_DATE_OF_BIRTH_ERROR', '&nbsp;<span class="errorText">(eg. 21/05/1970)</span>');
12. Pat yourself on the back, you're all done.