If you require other formatting than provided in the presets, you can edit it manually ([url:https://docs.oracle.com/javase/8/docs/api/java/text/SimpleDateFormat.html Formatting]). The timestamp can be further customized under "Chat Colors" and "Messages".

Append 'a:AM/PM' (including quotes) to customize AM/PM.

Stream Uptime

Add '{uptime}' (with the single quotes) to include the Stream Uptime. Optionally a character can be added before or after, separated by a comma, which will only be shown if the Uptime is displayed (e.g. '{/,uptime}').

Additional options can be specified, e.g. '{uptime:c}':

Modify Timestamp

Everything after ### is interpreted as a Custom Command that gets the formatted timestamp as it's $1- parameter. The result of the command is the resulting timestamp.