I have a worksheet that I enter time values into individual cells:

A1 = 6:00
B1 = 12:00
C1 = 12:45
D1 = 17:15

When I enter these values, I would like some of the values to round to the
next higher "30 minute increment" such as Cell C1 should become 13:00.

I would also like some of the values to round back to the next lower "30
minute increment" such as Cell D1 should become 17:00.

What function should I use, or what code should I type for this.

Darrell