safeik shtar mukdam - why is there a need for a "rosh hashana" for kings?

My son started learning Rosh haShana which forces me to try to keep pace with him. The first Mishna tells us that Rosh Chodesh Nisan was set as the Rosh haShana for kings. Documents used to be dated based on “year X of the king’s reign”; instead of using the king’s inauguration date, Chazal tell us to use Rosh Chodesh Nisan as the date to being counting a new year for all kings.

Why was there need for such a takanah? Some background: The gemara tells us that a post-dated shtar is acceptable, but a shtar dated before a loan occurred is pasul. A shtar creates a lien against all the debtors property; the creditor can even take property from a third-party who purchased it from the debtor after the loan occurred. A pre-dated shtar would give the creditor a lien against far more property than he is entitled too, and is therefore pasul. Tosfos holds such a shtar is completely invalid; Rashi holds it cannot be used to collect a lien, but it can be used to collect from free cash the debtor may have on hand. A loan post dated releases the debtor from part of the lien, so it is not a problem.

Rashi gives the following example to explain the need for the Mishna’s takanah: Reuvain has a shtar that says he loaned money to Shimon in Kislev of year 2 of the King’s reign. The witnesses testify that in reality the loan in question took place in Tamuz of year 2 of the King’s reign. Is the shtar a valid shtar or not? If the King’s reign were counted from his inauguration date, then if the King had been inaugurated in Tishrei (for example), then the shtar in pre-dated and pasul; if the King had been inaugurated in Adar (for example) the shtar is post-dated and OK. To remove the need to keep track of this variable and avoid confusion, Chazal said to use Rosh Chodesh Nisan as a uniform date.

Tosfos asks: in Rashi’s scenario we at worst have a doubt as to the validity of the shtar. Why would we reject such a shtar just because of a doubt? The gemara in Sanhedrin (32) writes that one cannot prove that witnesses are eidim zomimim because they signed a document dated for a time they could not possibly have seen a loan on (e.g. even if the shtar says the loan took place on Yom Kippur!) because we assume such a document is simply postdated – given the choice of rejecting the witnesses or assuming a shtar is postdated, we give them the benefit of the doubt. In Rashi’s scenario as well, given the choice of assuming the shtar is invalid or assuming it is postdated, why not simply give the creditor the benefit of the doubt? More to come…