[Samba] Hows samba calculating sambaPwdMustChange?

[Beast]

When samba password has been expired, user are force to change their password from client WS. 
Samba will modify sambaPwdMustChange attribute and the value seems always "2147483647", this not happen when changin password with smbpasswd.

>From where samba calculate value for "sambaPwdMustChange"? is it constant?
Is it possible to specify different value?

Tks.