However, you cannot set a signup deadline after the event; this rule is meant to prevent problems for users who rely on having the signup deadline enforced relative to the start time of the event. (For all-day events, the start time is considered to be 12:00am.)
However, we've had many requests for the ability to update the list of signups after an event has taken place. Our latest update makes this possible: You can now disable the signup deadline altogether.
You can disable the signup deadline for individual events in the event editor:
👩💻 Calendar admins: You can also disable the signup deadline for all future events in Settings > Event Fields > Signups.
With the deadline disabled, you can allow participants to signup at anytime during an event. You can also add signups to the list after an event for your own records.