Resolved the issue by changing the code in the function "string_custom_field_value" of custom_field_api.php to read
case CUSTOM_FIELD_TYPE_DATE:
if ($t_custom_field_value != null) {
if ($t_custom_field_value != " ") {
return date( config_get( 'short_date_format'), $t_custom_field_value) ;
}
}
break ;

Found that having empty custom date values also caused problems when displaying issue history. When updating at a point in workflow when the custom date field is required on update, the resultant bug view would show, just above the "Issue History" section:

SYSTEM WARNING: date() expects parameter 2 to be long, string given

Found the issue stemmed from the same issue, but the history_api.php file calls string_custom_field_value_for_email() instead of string_custom_field_value().

Resolved this issue by changing function string_custom_field_value_for_email in custom_field_api.php in the exact same fashion as dyawlak's change:

case CUSTOM_FIELD_TYPE_DATE:
    if ($p_value != null) {
        if ($p_value != " ") {
        return date( config_get( 'short_date_format' ), $p_value) ;
        }
    }
break ;

This issue is still present 1.1.2

Reproduced in 1.1.6

I believe this is a duplicate of the recently-resolved 0010184.