[turba] Birthday before 1970 not saved

[Wim]

Hello,
I have insert a field called "birtday" in turba. it is right shown in 
the adressbook, but when I try to save a date before 01 January 1970I 
got 0000-00-00 in the database field.
A date after 01 January 1970 works fine.

In attributes.php I have the field configured like:

$attributes['birthday'] = array(
     'label' => _("Birthday"),
     'type' => 'monthdayyear',
     'params' => array(1900, null, true, '%Y-%m-%d'),
     'required' => false,
);

Anyone an idea why this happen??
Thanks