Remote calendar - contents are not valid ICS format - Invalid property name DTSTAMP

[dasdundave]

The problem

After upgrading from 2025.5 to 2025.12 my work calendar no longer works.
It was configured as a Remote Calendar using an ICS URL from the HR website.

Tried removing the calendar and re-adding with the ICS URL, but could no add it again because of same error.

When manually downloading the ICS file, removing all DTSTAMP lines and placing the ICS file in the www folder, I can add it using this URL without errors: http://hass.myhome/local/Calendar.ics
This offcourse is not a solution because the ics file would not be updated.

Note: there is a space after DTSTAMP, but I've tried removing those first without succes:
DTSTAMP :20251017T073337Z EDIT: I must have made a mistake, because this is the issue!

What version of Home Assistant Core has the issue?

core-2025.12.2

What was the last working version of Home Assistant Core?

core-2025.05.x

What type of installation are you running?

Home Assistant OS

Integration causing the issue

Remote Calendar

Link to integration documentation on our website

https://www.home-assistant.io/integrations/remote_calendar/

Diagnostics information

No response

Example YAML snippet

Anything in the logs that might be useful for us?

2025-12-09 17:48:19.781 ERROR (MainThread) [homeassistant.components.remote_calendar.ics] Error parsing calendar information: Calendar contents are not valid ICS format, see the detailed_error for more information
2025-12-09 17:48:19.781 DEBUG (MainThread) [homeassistant.components.remote_calendar.ics] Additional calendar error detail: Invalid property name 'DTSTAMP '

Additional information

Redacted ICS data

BEGIN:VCALENDAR
VERSION:2.0
METHOD:PUBLISH
X-WR-CALNAME:REDACTED-NAME
X-WR-CALDESC:REDACTED-DESCRIPTION
PRODID:-//REDACTED-PROVIDER//REDACTED-NAME//NL
X-PUBLISHED-TTL:PT1M

BEGIN:VEVENT
UID:REDACTED-UID-1
DTSTAMP :20251017T073337Z
SEQUENCE:0
DTSTART:20241021T060000Z
DTEND:20241022T140000Z
SUMMARY:REDACTED-SUMMARY-1
DESCRIPTION:REDACTED-DESCRIPTION-1
STATUS:CONFIRMED
X-MICROSOFT-CDO-BUSYSTATUS:BUSY
X-MICROSOFT-CDO-ALLDAYEVENT:TRUE
X-FUNAMBOL-ALLDAY:TRUE
END:VEVENT

BEGIN:VEVENT
UID:REDACTED-UID-2
DTSTAMP :20251017T073337Z
SEQUENCE:0
DTSTART:20241024T060000Z
DTEND:20241024T150000Z
SUMMARY:REDACTED-SUMMARY-2
DESCRIPTION:REDACTED-DESCRIPTION-2
STATUS:CONFIRMED
X-MICROSOFT-CDO-BUSYSTATUS:BUSY
X-MICROSOFT-CDO-ALLDAYEVENT:TRUE
X-FUNAMBOL-ALLDAY:TRUE
END:VEVENT

BEGIN:VEVENT
UID:REDACTED-UID-3
DTSTAMP :20251017T073337Z
SEQUENCE:0
DTSTART:20241028T070000Z
DTEND:20241030T150000Z
SUMMARY:REDACTED-SUMMARY-3
DESCRIPTION:REDACTED-DESCRIPTION-3
STATUS:CONFIRMED
X-MICROSOFT-CDO-BUSYSTATUS:BUSY
X-MICROSOFT-CDO-ALLDAYEVENT:TRUE
X-FUNAMBOL-ALLDAY:TRUE
END:VEVENT

BEGIN:VEVENT
UID:REDACTED-UID-4
DTSTAMP:20251017T073337Z
SEQUENCE:0
DTSTART:20241105T070000Z
DTEND:20241106T150000Z
SUMMARY:REDACTED-SUMMARY-4
DESCRIPTION:REDACTED-DESCRIPTION-4
STATUS:CONFIRMED
X-MICROSOFT-CDO-BUSYSTATUS:BUSY
X-MICROSOFT-CDO-ALLDAYEVENT:TRUE
X-FUNAMBOL-ALLDAY:TRUE
END:VEVENT

END:VCALENDAR

[home-assistant]

Hey there @Thomas55555, @allenporter, mind taking a look at this issue as it has been labeled with an integration (remote_calendar) you are listed as a code owner for? Thanks!

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---

remote_calendar documentation
remote_calendar source
_{^{(message by IssueLinks)}}

[allenporter]

The error message is accurate "Error parsing calendar information: Calendar contents are not valid ICS format" and the calendar is not valid RFC5545 standard. You should file this issue report with your calendar provider for them to make valid calendar files as we can't really control that from home-assistant.

[dasdundave]

EDIT: I must have made a mistake in saving the file when I was testing. The root cause seems to be that space after DTSTAMP

After remove the space after each DTSTAMP occurance and I can add the ics file in HomeAssistant (http://hass.myhome/local/Calendar.ics).
DTSTAMP :20251017T073337Z --> DTSTAMP:20251017T073337Z

Whould it be a posibility in HASS to remove all spaces in the ICS file in the part before the ":"?

---

I agree that the responsibility for standards-compliant ICS output lies with the provider. If the ICS contains invalid properties, then technically the upstream source should correct that.

However, in practice it will be nearly impossible to get in touch with the provider of this ICS feed. And even if I manage that, there’s a good chance they won’t change anything — especially because the exact same ICS URL imports without issues in both Outlook and Google Calendar. From their perspective, the feed “works,” which makes it hard to convince them that anything needs to be fixed.

Given that other major calendar consumers accept the file, it might be worth considering whether Home Assistant’s parser could be made a bit more tolerant when encountering these common deviations in real-world ICS feeds.

Note: when I run the URL against on online validator, no errors popup concerning the DTSTAMP fields. I only get 2 warnings about 2 lines (description) that are slightly too long:

[allenporter]

A potential workaround is to import it into the providers that accept invalid calendars, then use the native integrations with home assistant, or determine if they re-write when exporting as ics.

There is some precedent for relaxing validations for things like microsofts published quirks that are non-standard or some of Google calendars bugs behind the compat mode flag though those have been targeted at widely used invalid bugs to limit the the number of hacks to maintain in the upstream library.

[AlyoshinAleksey]

Error adding entity calendar.o_calendar for domain calendar with platform remote_calendar
Traceback (most recent call last):
File "/usr/src/homeassistant/homeassistant/components/calendar/init.py", line 406, in post_init
CALENDAR_EVENT_SCHEMA(dataclasses.asdict(self, dict_factory=skip_none))
~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.13/site-packages/voluptuous/schema_builder.py", line 205, in call
return self._compiled([], data)
~~~~~~~~~~~~~~^^^^^^^^^^
File "/usr/local/lib/python3.13/site-packages/voluptuous/validators.py", line 256, in _run
return self._exec(self._compiled, value, path)
~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.13/site-packages/voluptuous/validators.py", line 386, in _exec
raise e if self.msg is None else AllInvalid(self.msg, path=path)
File "/usr/local/lib/python3.13/site-packages/voluptuous/validators.py", line 384, in _exec
v = func(path, v)
File "/usr/local/lib/python3.13/site-packages/voluptuous/schema_builder.py", line 549, in validate_dict
return base_validate(path, data.items(), out)
File "/usr/local/lib/python3.13/site-packages/voluptuous/schema_builder.py", line 382, in validate_mapping
raise er.MultipleInvalid(errors)
voluptuous.error.MultipleInvalid: Invalid rrule 'FREQ=WEEKLY;BYDAY=SU;WKST=MONDAY': invalid 'WKST': MONDAY for dictionary value @ data['rrule']

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/usr/src/homeassistant/homeassistant/helpers/entity_platform.py", line 1002, in _async_add_entity
await entity.add_to_platform_finish()
File "/usr/src/homeassistant/homeassistant/helpers/entity.py", line 1380, in add_to_platform_finish
self.async_write_ha_state()
~~~~~~~~~~~~~~~~~~~~~~~~~^^
File "/usr/src/homeassistant/homeassistant/components/calendar/init.py", line 561, in async_write_ha_state
super().async_write_ha_state()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
File "/usr/src/homeassistant/homeassistant/helpers/entity.py", line 1024, in async_write_ha_state
self._async_write_ha_state()
~~~~~~~~~~~~~~~~~~~~~~~~~~^^
File "/usr/src/homeassistant/homeassistant/helpers/entity.py", line 1149, in _async_write_ha_state
self.__async_calculate_state()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
File "/usr/src/homeassistant/homeassistant/helpers/entity.py", line 1086, in __async_calculate_state
state = self._stringify_state(available)
File "/usr/src/homeassistant/homeassistant/helpers/entity.py", line 1030, in _stringify_state
if (state := self.state) is None:
^^^^^^^^^^
File "/usr/src/homeassistant/homeassistant/components/calendar/init.py", line 544, in state
if (event := self.event) is None:
^^^^^^^^^^
File "/usr/src/homeassistant/homeassistant/components/remote_calendar/calendar.py", line 62, in event
return _get_calendar_event(event)
File "/usr/src/homeassistant/homeassistant/components/remote_calendar/calendar.py", line 100, in _get_calendar_event
return CalendarEvent(
summary=event.summary or "",
...<14 lines>...
location=event.location,
)
File "", line 11, in init
File "/usr/src/homeassistant/homeassistant/components/calendar/init.py", line 408, in post_init
raise HomeAssistantError(
f"Failed to validate CalendarEvent: {err}"
) from err
homeassistant.exceptions.HomeAssistantError: Failed to validate CalendarEvent: Invalid rrule 'FREQ=WEEKLY;BYDAY=SU;WKST=MONDAY': invalid 'WKST': MONDAY for dictionary value @ data['rrule']

[allenporter]

@AlyoshinAleksey lets track with a new issue please since its not the same issue we are discussing here and may be a regression in a new version of home assistant. It will be helpful to have you fill out the issue report.

[dasdundave]

A potential workaround is to import it into the providers that accept invalid calendars, then use the native integrations with home assistant, or determine if they re-write when exporting as ics.

My workaround for now is using a shell command. I've sent a message to the ICS supplier in hopes they will fix it from their end.

# My work calendar stopped working;
# Root cause is a space after DTSTAMP
# More info in the Github ticket:
# https://github.com/home-assistant/core/issues/158482

# WORKAROUND
# Download the ICS file to HomeAssistant
# Remove the spaces
# HomeAssistant retrieves the local fixed ICS file in the www folder

# TO ADD THE CALENDAR TO HOMEASSISTANT
# Add Remote Calendar
# URL: http://localhost:8123/local/calendar.ics

##################################################
## Components/Sensors
##################################################
shell_command:
  calendar_ics_workaround: >-
    wget -qO /config/www/calendar_raw.ics
    "https://www.company.be/hrsskmo/feeds/Calendar.ics?guid=xxxxxxxx&lang=nl"
    && sed 's/DTSTAMP :/DTSTAMP:/g' /config/www/calendar_raw.ics > /config/www/calendar.ics

##################################################
## Automation
##################################################
automation:
  - id: calendar_ics_workaround
    alias: "ICS workaround"
    mode: single
    triggers:
      - trigger: time_pattern
        hours: /1
    actions:
      - action: shell_command.calendar_ics_workaround

[alfred-reynolds]

@AlyoshinAleksey lets track with a new issue please since its not the same issue we are discussing here and may be a regression in a new version of home assistant. It will be helpful to have you fill out the issue report.

I've opened issue 158953 for the WKST exception issue

[allenporter]

Closing since the upstream calendar is not following the spec

[dasdundave]

The ICS supplier has fixed the bug from their end. They have removed the space and now the ICS file is accepted without errors.

DTSTAMP:20260310T144119Z