If your custom field is a date type field (e.g. Date1, Start1, etc.) then the format is determined by the option setting under, File > Options > General > Date format, and that's the only way to change it.
However, if you still want your general format for dates to include the date, weekday and time but want your custom field to only show "mm/dd" or "mm/dd/yy" then you will need to create your custom field as text (i.e. Text1, etc.)