time-stamp-old-format-warn

time-stamp-old-format-warn is a variable defined in `time-stamp.el'.
Its value is

ask

Documentation:
Action if `time-stamp-format' is an old-style list.
If `error', the format is not used. If `ask', the user is queried about
using the time-stamp-format. If `warn', a warning is displayed.
If nil, no notification is given.

You can customize this variable.