It is not permissible to tamper with the date of certification, because that involves deceit, lying and trickery towards employers for no good reason.
If the companies have stipulated that they will only accept new graduates, then you have to abide by this condition and it is haraam to resort to trickery in order to get around it, because the Prophet (blessings and peace of Allah be upon him) said: The Muslims are bound by their conditions. Narrated by Abu Dawood, 3594; classed as saheeh by al-Albaani in Saheeh Abi Dawood.
That is also because of the general meaning of the evidence that deceit and lying are haraam.
We advise you to fear Allah, may He be exalted, and to do a lot of righteous deeds and think positively of the Sovereign, the Most Generous, Whose stores never run out, and all provision, health and success come from Him.
We ask Allah to bestow His bounty upon you and provide you with permissible, blessed work.
And Allah knows best.
Reproduced from Islam QA