The word " resign " has generally three meanings depending on context. One is to relinquish or to quit a job or an official position on one's own, the other is to yield without resistance and the third is to renounce something. The first and the last one have the element of voluntariness and the second one would imply one's inability to withstand something overpowering or a kind of forced submission or total surrender. Philosophically and legally, voluntariness refers to one's choice made out of free will as opposed to a decision made under duress or coercion.
Therefore, in the realm of employment, " resignation " would certainly fall under the first meaning only referring to the act of unilateral termination of the contract of employment, be it express or implied, by the employee.
Hence, firstly, predated or undated, if a resignation is obtained by the employer, it amounts to coercion for the choice given is not about resignation but about its timing only.
Secondly, resignation would take effect only from the date of formal relieving after its acceptance by the employer. Again it is also circumscribed by certain other conditions already stipulated in the employment contract such as due notice or payment in lieu of notice, compliance with the terms of employment bond if any, towards an early exit, repayment of loans and advances, if any obtained. If all such things are waived in a single stroke of acceptance, then the effect of resignation becomes instantaneous. Of course, it may be a gesture of magnanimity mixed with goodwill or a firm resolve on the part of the employer to get rid of an unwanted employee hassle-free.
Hope you got the point.
From India, Salem