MAMS User import date format

[webform]

I have a couple of User Properties as type Date (Birthday etc.) But most users fail to import with error message "Invalid value for column birthday".

How do i import users with a birthday older than 1970-01-01?

I've tried to convert the birthday to strtotime before import and this works for some users but not most.
A user with birthday "1941-04-04" converted to "-907113600" does import. But a user with birthday "1955-01-14" converted to "-472262400" does not