For times listing in column B, starting at B2:
Start by selecting column B and setting the cell format to:
Date and Time
Time: 19:08 OR 7:08 PM
Enter the starting time in B2.
Enter the formula below in B3, then Fill down as far as you need.
Be aware that the value in all of these cells is a Date and Time value, and includes the date current when the first entry was made as part of the value, efvgen though it is not displayed.
Thanks for the feedback, Leeloo. I was in a bit of a rush whne I posted that, and didn't have time to add the explanation below. I'm adding it now for the benefit of anyone looking for a similar solution, and wondering about the thinking behind this one,
The base unit of Date and Time values is the (24 hour) Day.
Adding 1 to a D&T value returns the D&T value one day later, at the same time of day.
Adding 1/2 to a D&T value returns the Date and time 12 hours (one-half day) later than the start time.
Leeloo wanted the time in 20 minute increments.
20 minutes is 20/60 of an hour, or reduced to lowest terms, 1/3 of an hour.
One hour is 1/24 of a day.
Combining those gives 1/24 * 20/60, or 20/(24*60), or, reduced to the terms shown above, 1/(24*3).
So the amount to be added to the previous value to give a Date and Time value 20 minutes later is 1/(24*3).