Contracts can also, confusingly, contain defined dates such as ‘commencement date’, ‘effective date’ or ‘start date’.
However, at common law this was a criminal offence (going by the contradictory sounding name of uttering a false document) and in most English law based legal systems it is still an offence today, although in many cases statutory provisions have superseded the common law (for example, in the British Virgin Islands see section 242 of the Criminal Code 1997).
If a contract does not specify its effective date, it goes into effect on the date it was signed by the person to whom the contract was offered for a signature.
Sometimes you will want the effective date to be different from the date of signing, either earlier (i.e., backdating) or later (i.e., predating).
Instead, it’s the company’s obligation to pay the employee, and the employee’s obligation to work for that pay, that commences later, and that’s what I’d say in the contract.
If you need a defined term to refer to that later day, I’d use something like used in a contract to refer to some date in the past.