erc-insert-timestamp-function

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

erc-insert-timestamp-left-and-right

• This variable may be risky if used as a file-local variable.

Documentation:
Function to use to insert timestamps.

It takes a single argument STRING which is the final string
which all text-properties already appended. This function only cares about
inserting this string at the right position. Narrowing is in effect
while it is called, so (point-min) and (point-max) determine the region to
operate on.

You will probably want to set
`erc-insert-away-timestamp-function' to the same value.

You can customize this variable.