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"Repeat every X weeks" is broken in certain time zones #408

Open
7 tasks done
tom93 opened this issue Jan 4, 2025 · 0 comments · May be fixed by #409
Open
7 tasks done

"Repeat every X weeks" is broken in certain time zones #408

tom93 opened this issue Jan 4, 2025 · 0 comments · May be fixed by #409
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tom93 commented Jan 4, 2025 •
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Proposed fix: #409

Checklist

  • I can reproduce the bug with the latest version given here.
  • I made sure that there are no existing issues - open or closed - to which I could contribute my information.
  • I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
  • I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
  • I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
  • This issue contains only one bug.
  • I have read and understood the contribution guidelines.

Affected app version

1.1.0

Affected Android/Custom ROM version

Android 14

Affected device model

Emulator

How did you install the app?

GitHub releases

Steps to reproduce the bug

  1. Set the time zone to Antarctica/McMurdo
  2. Create an event on Monday 11 March 2024 at 9am with custom repetition every 3 weeks
  3. View the occurrences of the event

Expected behavior

The occurrences should be:
Mon 11 Mar 2024
Mon 01 Apr 2024
Mon 22 Apr 2024
Mon 13 May 2024

Actual behavior

The occurrences are:
Mon 11 Mar 2024
Mon 01 Apr 2024
Mon 15 Apr 2024 (incorrect -- gap is 2 weeks instead of 3)
Mon 06 May 2024

Screenshots/Screen recordings

1-set-time-zone 2-event-details 3-occurrences-march 4-occurrences-april

Observe how in the final screenshot (April) the occurrences are incorrect.

Analysis

The bug is in the method isOnProperWeek in models/Event.k.

It calculates the week number of a timestamp by taking the start-of-day (in the default time zone) as a Unix timestamp then dividing by 7 weeks and rounding to the nearest integer. This works in most cases, but is actually wrong. It mostly works because:

  • The Unix epoch is Thursday 00:00 UTC.
  • So timestamps within 3.5 days of Thu 00:00 UTC are rounded to the same week number.
  • So the threshold is Sunday 12:00 UTC.
  • In most time zones, the start-of-day of Sunday is before Sunday 12:00 UTC, and the start-of-day of Monday is after Sunday 12:00 UTC. So in most time zones the calculation assigns the same week number to Mon-Sun.

However, this breaks in time zones such as Antarctica/McMurdo, because in that time zone the start-of-day of Monday is very close to Sunday 12:00 UTC, so the rounding is unstable, and daylight saving causes the calculation to produce an incorrect week number.
@tom93 tom93 added bug Something is not working needs triage Issue is not yet ready for PR authors to take up labels Jan 4, 2025
tom93 added a commit to tom93/Calendar that referenced this issue Jan 4, 2025
@tom93
Fix calculation of events repeating every X weeks
27fcc6a 
The previous implementation was broken on some time zones (see FossifyOrg#408
for details).

The new implementation hard-codes the start of the week to be Monday
to match the previous behaviour. Ideally this should be configurable
(see the TODO comment for more notes).

Fixes FossifyOrg#408.
@tom93 tom93 linked a pull request Jan 4, 2025 that will close this issue
Open
4 tasks
tom93 added a commit to tom93/Calendar that referenced this issue Jan 4, 2025
@tom93
Fix calculation of events repeating every X weeks
18610fa 
The previous implementation was broken in some time zones (see FossifyOrg#408
for details).

The new implementation hard-codes the start of the week to be Monday
to match the previous behaviour. Ideally this should be configurable
(see the TODO comment for more notes).

Fixes FossifyOrg#408.
@Aga-C Aga-C removed the needs triage Issue is not yet ready for PR authors to take up label Jan 4, 2025
tom93 added a commit to tom93/Calendar that referenced this issue Jan 16, 2025
@tom93
Fix calculation of events repeating every X weeks
fa91af0 
The previous implementation was broken in some time zones (see FossifyOrg#408
for details).

The new implementation hard-codes the start of the week to be Monday
to match the previous behaviour. Ideally this should be configurable
(see the TODO comment for more notes).

Fixes FossifyOrg#408.
tom93 added a commit to tom93/Calendar that referenced this issue Jan 18, 2025
@tom93
Fix calculation of events repeating every X weeks
f16fc71 
The previous implementation was broken in some time zones (see FossifyOrg#408
for details).

The new implementation hard-codes the start of the week to be Monday
to match the previous behaviour. Ideally this should be configurable
(see the comments in the code for more notes).

Fixes FossifyOrg#408.
tom93 added a commit to tom93/Calendar that referenced this issue Jan 18, 2025
@tom93
Fix calculation of events repeating every X weeks
530e145 
The previous implementation was broken in some time zones (see FossifyOrg#408
for details).

The new implementation hard-codes the start of the week to be Monday
to match the previous behaviour. Ideally this should be configurable
(see the comments in the code for more notes).

Fixes FossifyOrg#408.
tom93 added a commit to tom93/Calendar that referenced this issue Jan 18, 2025
@tom93
Fix calculation of events repeating every X weeks
2298a4d 
The previous implementation was broken in some time zones (see FossifyOrg#408
for details).

The new implementation hard-codes the start of the week to be Monday
to match the previous behaviour. Ideally this should be configurable
(see the comments in the code for more notes).

Fixes FossifyOrg#408.
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Fix calculation of events repeating every X weeks tom93/Calendar
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