The longstop date is the date in an Australian share sale or asset sale agreement after which, if completion has not occurred, either party (or only the benefitting party) typically has the right to terminate the deal. Longstop dates in Australian sub-$20M deals are usually set 60 to 120 days after signing, with the period chosen to allow time for conditions precedent (regulatory approvals, third-party consents, financing) to be satisfied.
The longstop date should be long enough to accommodate realistic delays in any conditions precedent, but short enough that neither party is held in the deal indefinitely. Where ACCC merger clearance or FIRB approval is required, the longstop date should be calibrated to the statutory timelines for those approvals.