erc-timestamp-format-left

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

"
[%a %b %e %Y]
"

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".

This timestamp is used for timestamps on the left side of the
screen when `erc-insert-timestamp-function' is set to
`erc-insert-timestamp-left-and-right'.

If nil, timestamping is turned off.

You can customize this variable.