Gittin 7:7

If a man divorces his wife saying, “Here is your get on the condition that I do not return within 30 days,” and he was traveling from Judea to the Galilee and he reached the border city of Antipatris, after which he returned, the get is voided. If he was traveling from the Galilee to Judea and he reached the border city of Kfar Osnai, after which he returned, the get is voided. If he was traveling overseas and he reached the border city of Akko, after which he returned, the get is voided. If he said, “Here is your get on the condition that I remain away from your face for 30 days,” then even if he came and went several times, the get is valid because he wasn’t in her presence.

Gittin 7:8

If he says, “Here is your get on the condition that I do not return within 12 months” and he dies within the 12 months, then the get is invalid. If he says, “Here is your get effective now on the condition that I do not return within 12 months” and he dies within the 12 months, then the get is valid.