Hello... I am kinda new to all this.. I hope I can find my way back to
see if anyone can help.

What I am trying to do is have an excel file that currently displays
Time like this:

A B
5:45 AM
6:15 AM
12:55 PM
1:15 AM

To do this : I need the time rounded down to the last half hour so
that in column C it would take column A1 which is 5:45 to be displayed
in column C1 as 5:30 A2 would be C2 6:00.

A B C D
5:45 AM ?? 5:30 5:30
6:15 AM ?? 6:00 6:00
7:28 PM ?? 7:00 19:00
12:55 PM ?? 12:30 24:30
1:15 AM ?? 1:00 1:00