Yevamos 31

The Gemara said they decreed to write the date on a slave's document. Rashi explains this to mean a slave's sale document. Tosfos asks, if so, the could have asked from a document for selling land. Furthermore, the nrabbis never made a decree to date those documents. So Tosfos explains that the Gemara refers to documents of freeing slaves. You need to have a date to know when he had a Halacha of a slave and when he had a Halacha of a regular Yisrael regarding when he was permitted to marry a Jewish woman. Rabbi Chaim Smulowitz If you ever though that Tosfos was beyond you. Think again. A few more free copies of Gemara and Tosfos: Sukka 2a-11a is still available at tosfos.ecwid.com