How do I prevent Excel from adding a date when custom formatting cells for hours and minutes?

Date & Time in Numbers always has a date and a time even if you enter "just the time" and format the cell not to display a date. If you don't supply a date Numbers guesses that you mean the date on which you entered the value, helpful in many situations but not so helpful for what you want to do.

An easy workaround is to format the cells as Text and then enter the times. The values will display as left-aligned by default, but you can manually change the alignment if needed.

Numbers will happily perform "date arithmetic" on such "text times", e.g. subtracting one from the other to get the duration.

SG

You mention Excel, but show a Numbers spreadsheet. Which application is it you're working in?

Assuming Numbers, there is a specific data type - Duration - which is designed just for this purpose - a number of hours/minutes/seconds (or days, weeks... etc.), so maybe consider using that for your table?

In either case, date fields always inherently contain a date and a time. how you choose to display that is independent of the underlying cell value, and there are numerous Date & Time functions that can be used to extract the data you want from them.