While Twisted offers facilities to call a Python callable after a
specified time interval (i.e. call this function in 5 seconds), it does not
offer any way to make a call at a specified time (call this function at
5:00pm every weekday). This functionality is often desired (or required)
in long-running processes like most Twisted daemons.

txScheduler offers just this functionality, allowing scheduling of Python
tasks from within a Twisted app according to arbitrarily complex rules
specified by the developer.