There are a number of questions that may be asked on Rashi's explanation.
(a) According to Rashi, who explains that when the husband appeases the wife
we are afraid that the Get might be a Get Yashan and the date of the Get
might precede the date of the child's conception, why are we permitted to
deliver the Get when the husband announces that he trusts his wife's
testimony to say whether or not he appeased her? Even if he trusts her,
*Beis Din* cannot trust her, and therefore Beis Din should be concerned for
the welfare of a child who was conceived after the date written in the Get,
despite the woman's testimony.
(b) If the problem of "Shema Piyes" is that the husband might have lived
with his wife before the Get took effect, then how can this concern be
applied in a case where the husband gives a Get stipulating that it should
take effect in twelve months if he does not return before the twelve months
passes? If the husband was together with his wife, then the Gemara should be
concerned not that the Get is a Get Yashan, but that it is Batel altogether,
since by returning to his wife before twelve months passes, the condition
that he stipulated was not fulfilled and the Get cannot take effect at all!
These questions are asked by TOSFOS (18b, DH Shema Piyes).
(a) TOSFOS there asks another question on Rashi from the Gemara later (76b).
The Beraisa there teaches that in a case of a Get that takes effect after
thirty days, where the husband was not together with his wife during that
time, we are not afraid that the Get is a Get Yashan. The Gemara asks that
we should still be concerned that "Shema Piyes," that he appeased her.
However, if the Beraisa just said that there is no reason to be concerned
that the Get is a Get Yashan, then why is the Gemara concerned that he
It is clear from the Gemara there that the concern for Get Yashan and for
"Shema Piyes" are two completely different Halachos. Rashi there (DH Shema
Piyes) answers this question by explaining that Beis Din certainly has no
reason to be concerned that the husband clandestinely secluded himself with
his wife and the date in the get will predate the conception of their child.
However, we should be concerned that the husband -- in an attempt to
frustrate his wife -- will come at a later date and try to invalidate the
Get by *claiming* that he secluded himself with her and that the Get is a
Get Yashan! Just like the Chachamim took precautions to prevent the husband
from saying that the Get he gave is forged when he sends it from Medinas
ha'Yam, the were also concerned that the husband should not be able to say
that it is a Get Yashan. Hence, it is obvious why we have no need to be
concerned when the husband announces that he trusts his wife's testimony:
since he gives her the power to override whatever he says, we are no longer
concerned that he will claim that the Get is a Get Yashan.
Tosfos himself (18b) in the name of the RI suggests that this is Rashi's
(Although there is an opinion in the Gemara on 79b that if the husband
already divorced his wife with a Get Yashan, she is permitted to marry
someone else l'Chatchilah on the basis of that Get, nevertheless Rashi there
(DH Im Nisgarshah) explains that she may only get married on the basis of
the Get when her husband has left to Medinas ha'Yam and she cannot get
another Get from him. Our concern here is that the husband might claim that
the Get is a Get Yashan and prevent her from marrying as long as he is in
town. See also TOSFOS in Yevamos 14b, DH b'Get Yashan.)
(b) Apparently, Rashi learns that when the husband makes a condition stating
that the Get will take effect only "if I do not return within twelve
months," his condition means that the Get will take effect only if he does
not return *permanently* within twelve months. Coming back to visit during
that time is *not* a contradiction to his condition and does *not* annul the
ability for the Get to take effect after twelve months! Hence, the Get can
take effect after twelve months if not for the problem of Get Yashan.
(RABEINU TAM in Tosfos (18b) suggests another approach to the Sugya of
"Shema Piyes" because of these questions.)