If your buyers have a backup contract with a termination option, Paragraph D of the Addendum for “Back-Up” Contract (TAR 1909, TREC 11-7) explains the start and end of that option period.
The addendum says that the time for giving notice of termination begins on the effective date of the backup contract. It continues without restriction until the amended effective date of the contract—the day your buyers receive notice of the first contract’s termination. At that point, the option period lasts for the time indicated in the contract.
For example, the effective date of your buyers’ backup contract with a 10-day termination option is December 1. If they become the primary contract on December 15, they have the unrestricted right to terminate from December 1 until December 25.