Related Questions:
Edit a Booking to have an empty date
Change a Booking date without knowing the expected date
Answer:
Open the Booking you wish to edit
At the top of the Booking Summary page click on the Booking Dates (under the Booking Name, above the pipeline.)
Click on the Start Date
Press the red Clear button
Click on the End Date
Press the red Clear button
Press Save
Confirm the Booking now says "Date Unspecified"