You can schedule a webform to open or close on specific dates and times. Logged in, go to the form and click Edit. There's a section for "Webform settings." Change the status to "Scheduled" and set either, or both, date fields.

This is useful if you want a form to be open to the public (for example) Monday at 8am, or if your webform is related to an event and you want to stop accepting submissions after (for example) Friday at 5pm.