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You can get the full hours before / after a certain time stamp by using the FLOOR / CEILING functions with the significance of 1/24. Excel handles dates and times as numbers where the whole numbers stand for the date part and decimals as hours. Since there's 24 hours in a day 1/24 equals 1 hour.
FLOOR & CEILING functions round the given number down & up to the nearest specified multiple. The results from these formulas are just numbers.
If you just wanted to use the earlier full hour or later full hour you could just enter the FLOOR or CEILING to a column and use number formatting to display the results as times but since you want to have two results in a single cell you're going to have to wrap each formula in a TEXT function that formats the number the way you want to.
When you combine all of the above the formula becomes something like:
Excel Formula:
=TEXT(FLOOR(B2,1/24),"h:mm-")&TEXT(CEILING(B2+0,0001,1/24),"h:mm")