The "!" is well used to signify NOT or, from the time.conf man page[1] "anything but". The same information is also available from the docs for pam_time at kernel.org[2].

I'm currently using Ubuntu 7.04 and similar information is easily available in time.conf. Simply use less or cat to display the contents of the file:

# was initially based heavily on that of the shadow package (shadow-960129).
#
# the syntax of the lines is as follows:
#
# services;ttys;users;times
#
# white space is ignored and lines maybe extended with '\\n' (escaped
# newlines). As should be clear from reading these comments,
# text following a '#' is ignored to the end of the line.
#
# the combination of individual users/terminals etc is a logic list
# namely individual tokens that are optionally prefixed with '!' (logical
# not) and separated with '&' (logical and) and '|' (logical or).
#
# services
# is a logic list of PAM service names that the rule applies to.
#
# ttys
# is a logic list of terminal names that this rule applies to.
#
# users
# is a logic list of users to whom this rule applies.
#
# NB. For these items the simple wildcard '*' may be used only once.
#
# times
# the format here is a logic list of day/time-range
# entries the days are specified by a sequence of two character
# entries, MoTuSa for example is Monday Tuesday and Saturday. Note
# that repeated days are unset MoMo = no day, and MoWk = all weekdays
# bar Monday. The two character combinations accepted are
#
# Mo Tu We Th Fr Sa Su Wk Wd Al
#
# the last two being week-end days and all 7 days of the week
# respectively. As a final example, AlFr means all days except Friday.
#
# each day/time-range can be prefixed with a '!' to indicate "anything
# but"
#
# The time-range part is two 24-hour times HHMM separated by a hyphen
# indicating the start and finish time (if the finish time is smaller
# than the start time it is deemed to apply on the following day).
#
# for a rule to be active, ALL of service+ttys+users must be satisfied
# by the applying process.
#

#
# Here is a simple example: running blank on tty* (any ttyXXX device),
# the users 'you' and 'me' are denied service all of the time
#

#blank;tty* & !ttyp*;you|me;!Al0000-2400

# Another silly example, user 'root' is denied xsh access
# from pseudo terminals at the weekend and on mondays.

#xsh;ttyp*;root;!WdMo0000-2400

#
# End of example file.



[1] http://linux.die.net/man/5/time.conf
[2] http://www.kernel.org/pub/linux/libs/pam/Linux-PAM-html/sag-pam_time.html