I'm not sure I understand the question, but let's clarify a few basic points:

1) The choice of Nisan aand Tishrei as examples are the Gemora's choice and not Rashi's addition to the explanation

2) We are not discussing here a counterfeit Shtar, that is to say one that was "wrote and signed in Tishrei, predated to Nissan," but rather was written with the correct date but the actual loan was delayed and the Shtar becomes predated by default and not by design.

3) The Gemora at this point is referencing a Din that it is permitted to write a Shtar for the borrower even though the lender is not present (thereby creating the risk of a delayed loan). Since this is the Din we should always be wary of such a problem, not only when the Shtar has fallen. On this the Gemora presents two approaches, Rav Asi and Abayei. Rav Asi says that this Din only applies to Shtaros that reference the fact (with an explicit stipulation) that with the signing of the Shtar the associated liens were established, thereby making the date of the ultimate cash loan irrelevant. Abayei states that even if the Shtar does not state such a Kinyan, but it is not the passage of money from hand to hand that creates the lien but rather the signing of the Shtar that creates the lien, so ultimately the date of the delivery on the cash of the loan is irrelevant. In essence, both Rav Asi and Abayei have given the same answer, except that according to Abayei it is an automatic result of the witnesses signing, and according to Rav Asi it is only so if the is a stipulation of such lien mentioned explicitly in the Shtar.

Hope this is helpful, if I have missed the point, please send me a follow-up.

Shimon Brodie

The question is on the second answer of Rashi 13a DH Hani Mili.

Rashi asked:- how can Abaye say that the witnesses who signed on the Shtar give the Shibud to the creditor even though they signed before the loan was made? The problem is that the Mishnah Shevi'is 10:5 states that a Shtar which carries a date from before the loan was made is invalid. We see that the date must not be before the loan, so how can Abaye say that they signed before the loan?!

In his second answer to this question ("Ee Nami") Rashi says that the Shtar Mukdam, referred to in Sheviis 10:5, was written and signed in Tishrei, but the date written on it was Nisan, which is the New Year for Shtaros. The writing, signing and the loan was in Tishrei but the date was Nisan, so this is Mukdam. Abaye's shtar is different; the date on it is the same date that the witnesses signed, but the loan was not made until later.

The reason that Rashi writes that 1st Nisan is the New Year for Shtaros, is to make sure that this Shtar was written before the loan was made. For Shtaros, the first day of the year is 1st Nisan. It follows that if we see a Shtar with the date Nisan on it; and the loan was made in Tishrei; this must be an early shtar (Shtar Mukdam) it cannot be a late Shtar (Shtar Meuchar). Let us say, for example, that the date in the Shtar is Nisan in the year 205 of the Shtar counting. This must be before Tishrei of year 205.

This is stated by Chochmas Manoach, printed at the back of the Gemara. He writes that since it states Nisan in the Shtar and it also states the number of the year, it follows that it must have been written at the beginning of the year, so it must be early. The Rashash writes something similar; that if the New Year for Shtaros would have been Tishrei or one of the summer months, then it would be Meuchar if it would bear the date Nisan, but now that Nisan is the first month of the year of Shtaros, this means it is Mukdam.

A Freilicher Purim!

Dovid Bloom