ר"ש is not concerned for שמא יחפה, and maintains that the בעל forfeits the פירות from the time the גט is written, and עדים do not sign a predated גט unless the verify that it was written on that date.
Why did they institute a date by גיטין, etc. - מפני מה תיקנו זמן בגיטין
A woman is believed as to when she was divorced, regarding מזונות. A husband is believed that he divorced his wife, only regarding the future, provided his wife does not deny it. עדיו בחתומיו זכין לו only if there is זמן in the שטר. They were מתקן זמן by גט שחרור so the עבד should not challenge his new owner.
ר"ל said because of fruits - ריש לקיש אמר משום פירות
By a גט מאוחר the woman becomes divorced at the date written in the שטר and the בעל has the פירות until then. ר"ש maintains that ראוי לגרש is sufficient to render the גט כשר , while the רבנן maintain that it also requires a קול.