Hi istril
There is no number format that can directly diplay 18:00 as 6:00, they are not, in fact, the same time.
You'll have to find some workaround that may suit you. For ex., 2 options:
1 -use in another cell a formula, like
=SUBSTITUTE(SUBSTITUTE(TEXT(A1,"h:mmAM/PM"),"AM",""),"PM","")
Notice that the result in the cell is a string, not a time value, and you lose the information if it is am/pm
2 - hide the am/pm part in a second line
You can format the cell with a format this way.´
In the Custom Format write
. h:mmAM/PM
. position the cursor between the h:mm and the AM/PM
. press CTRL-J
Format the cell with wrap text
This format sends the AM/PM part to a second line. By keeping the default row height, this second line is not visible and so it's like you have the format you wanted.
Notice that in this case you still have the time value in the cell, you lost no information.