英语翻译合同签订日期以双方中最后一方签署并加盖公章或合同专用章的日期为准.

The two sides signed the contract date to the last party to the contract signed and stamped with official seal or seal of the closing date.

“双方”在合同中一般要事先约明,一般是“Parties”或者“Party A and Party B（甲方和乙方）”.
因此可以翻译如下（意译）：The contract shall become effective by the official seal or contract seal of Party A or Party B whichever signs later.