Is it permissible to pay the zakat on one's salary on its former due date, or on the date it was paid last year?

It is permissible to advance the payment of zakat before the completion of the hawl (zakat year) for someone who possesses the nisab (minimum threshold), and this does not change the original time of the hawl. For example, if the hawl is due in Rabi' al-Akhir and the zakat is advanced in Rabi' al-Awwal, this does not obligate paying it in Rabi' al-Awwal permanently thereafter. Nevertheless, there is no objection to shifting the beginning of the hawl and settling on it if that is easier for the muzakki (person paying zakat).