erc-server-timestamp-format

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

"%Y-%m-%d %T"

Documentation:
Timestamp format used with server response messages.
This string is processed using `format-time-string'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.3 of Emacs.