I’m not sure this is right, but this is where I’d start solving this problem.
Airtable doesn’t have time only fields. I don’t know why. But it does have a lot of cool Date/Time formulas that will help you here.
So in your formula, I’d suggest you do the following.
Extract the hour
Extract the minutes
Use the minutes’ value to divide by 50 and then use
ROUND(value, precision) to round to the value you’re looking for
Hope that helps!
Here’s the reference sheet in case you need it