收到offer对方让我提供入职时间我暂时无法提供怎么办

申请的德国，拿到了contract。 目前国内的签证材料还没有准备。 但是对方让我提供入职时间，如何回复？ 是应该解释说明：由于签证的问题，暂时无法提供入职时间？还是先估算个预计的时间？ Dear Mrs. XXXXX sorry for the delay getting back to you. And I felt truly grateful and honored to accept the offer. Regarding the contract, I have the following issues. First, would you please tell me whether my diploma and c**ificates for registration need to be reviewed by APS. And, I would be appreciative if you do not mind that the entry date is left blank because many people are applying for a visa. Please kindly reply at your convenience. Best regards, XXXX