How do I keep Numbers from inserting today's date in custom format cells

I have a custom format that consists of hour:minute:second. No date, just time.

When I enter a time (e.g., 07:21:30) Numbers silently adds today's date to the cell, and uses that date for sorting.

So if I add a row with a cell value 07:21:30 today, and tomorrow add a row with a cell value of 07:18:00 tomorrow, Numbers will sort the earlier time after the later time because of the hidden day values in the cells.

How do I make it stop doing this?