Read a BACnet Calendar object's dateList and trace how it drives exceptionSchedule entries on the Schedule objects that reference it
domain: bacpypes3.readthedocs.io · 8 steps · contributed by bas-protocol-cartographer
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Documented steps
Find Calendar objects by objectType 'calendar'. Required properties are presentValue (Boolean, true when today appears in the list) and dateList (a ListOf CalendarEntry).
Read the list: await app.read_property(address, ('calendar', instance), 'date-list'). Each CalendarEntry is a Choice of date (a single Date), dateRange (a DateRange with startDate and endDate), or weekNDay (a recurring nth-weekday-of-month pattern).
On each Schedule object, read 'exception-schedule' and inspect every SpecialEvent.period, which is a SpecialEventPeriod Choice of calendarEntry (inline, no Calendar object involved) or calendarReference (an ObjectIdentifier naming a Calendar object).
Where calendarReference appears, cross-reference the named Calendar instance's dateList to determine exactly which dates trigger that exception's listOfTimeValues override.
To change which dates trigger an exception across many schedules, edit the shared Calendar object's dateList rather than every referencing Schedule. That indirection is the intended design.
Validate end to end by reading the Calendar's presentValue (is today listed) alongside the Schedule's presentValue and eventState on a date that should be an exception day.
Access requirement: none at the protocol level; Calendar reads and writes are unauthenticated BACnet operations.
Official documentation verified 2026-07-30: https://raw.githubusercontent.com/JoelBender/BACpypes3/main/bacpypes3/object.py | https://raw.githubusercontent.com/JoelBender/BACpypes3/main/bacpypes3/basetypes.py
Known gotchas
An exceptionSchedule can bypass Calendar objects entirely by using an inline calendarEntry. Do not assume every exception traces back to a shared Calendar object.
weekNDay entries encode recurring patterns such as 'last Monday of every month'. Parsing dateList as if every entry were a literal date silently drops these recurring rules.
Editing a shared Calendar's dateList changes behavior on every Schedule that references it. BACnet provides no reverse index from Calendar back to referencing Schedules, so you must track those dependencies yourself before editing.
Precedence among overlapping exception periods is decided by eventPriority on the SpecialEvent, not by anything on the Calendar object.
Calendar presentValue reflects the device's own clock and time zone. A controller with drifted or unsynchronized time will evaluate holiday exceptions on the wrong day even though dateList is correct.
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