This setting will only affect one instance of the date time or time field and will not affect report rendering.
To build a new format simply type out the format that you would like to see using the special part markers where you want the different date parts to appear. For example a 12 hour time format would be {g}:{i} {a} and would output something like 3:36 pm.
Available Time Parts
AMPM Lowercase {a} - Lowercase Ante meridiem and Post meridiem, am or pm.
AMPM Upppercase {A} - Uppercase Ante meridiem and Post meridiem, AM or PM.
Hour {g} - 12-hour format of an hour without leading zeros, 1 through 12.
Hour 24 {G} - 24-hour format of an hour without leading zeros, 0 through 23.
Hour 24 with Leading Zero {H} - 24-hour format of an hour with leading zeros, 00 through 23.
Hour with Leading Zero {h} - 12-hour format of an hour with leading zeros, 01 through 12.
Minute {i} - Minutes with leading zeros, 00 through 59.
Second {s} - Seconds, with leading zeros, 00 through 59.