Timezone startOf DST off-by-one

In Australia DST ends on the 4th of April 2021.

Using the timezone plugin and v1.10.4 (latest):

• Getting the start of the day before this date (in DST) returns the correct value
• Getting the start of the day after this date (outside DST) returns an incorrect value.

dayjs.tz('2021-04-01 08:00', 'Australia/Melbourne').startOf('day').format()
// 2021-04-01T00:00:00+11:00 (correct)

dayjs.tz('2021-04-15 08:00', 'Australia/Melbourne').startOf('day').format()
// 2021-04-14T23:00:00+11:00 (incorrect)

Curiously, the changelog mentions this issue was fixed in v1.9.7, so I switched to 1.9.6 to test this and it seems to be working correctly:

dayjs.tz('2021-04-01 08:00', 'Australia/Melbourne').startOf('day').format()
// 2021-04-01T00:00:00+11:00 (correct)

dayjs.tz('2021-04-15 08:00', 'Australia/Melbourne').startOf('day').format()
// 2021-04-15T00:00:00+11:00 (correct)

Is anyone up for fixing this? I’ll throw a $100 bounty down. Surely there are some others who also need this fixed and want to add to the pool!

[ashconnell]

@apm78 day-js timezone support is insanely broken.

The docs make it seem like timezone support is production ready but it’s literally a single ~80 line file that barely covers anything.

I’m not very knowledgeable with timezones so it’s hard for me to contribute, but strategically I think this repo should:

1. Make it blatantly obvious that timezone support isnt ready to be used
2. Build a test suite that cross checks results with something that is production ready, eg Moment.js

I spent literally weeks adding day-js to a new platform before realising these issues existed and then spent literally days swapping it back out for Moment.js

[nemphys]

@iamkun do you have any ETA plans for a new release? The dev branch contains a commit that fixes a pretty bad timezone issue with DST. AFAICT there’s no historical release that has stable timezones except the upcoming one.

+1 on this, would have to have to revert to moment.js because of this!