erc-away-timestamp-format

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

"<%H:%M>"

Documentation:
Timestamp format used when marked as being away.

If nil, timestamping is turned off when away unless `erc-timestamp-format'
is set.

If `erc-timestamp-format' is set, this will not be used.

You can customize this variable.