erc-echo-timestamp-format

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

"Timestamped %A, %H:%M:%S"

Documentation:
Format string to be used when `erc-echo-timestamps' is non-nil.
This string specifies the format of the timestamp being echoed in
the minibuffer.

You can customize this variable.