erc-timestamp-format

erc-timestamp-format is a variable defined in `erc-stamp.el'.
Its value is

"[%H:%M]"

Documentation:
If set to a string, messages will be timestamped.
This string is processed using `format-time-string'.
Good examples are "%T" and "%H:%M".

If nil, timestamping is turned off.

You can customize this variable.