Session time input changes time unexpectedly

[ryelle]

The session time input is not working correctly, something is causing it to jump around while entering times. This appears to only happen if the session is after daylight saving time ends.

In the video, you can see the number is skipping down by two despite me only hitting the down arrow once. Then I hit up, and it doesn't always change. Moving over to toggle AM/PM, the time also changes. Back to the input, entering 12 automatically updates to 11.

ScreenFlow.mp4

Video recorded on MacOS + firefox.

Thanks to platcatheo for reporting this in Slack.

[ryelle]

This is to do with how the timezone offset is calculated, as a fixed amount for a given location rather than date-aware (for example, if the timezone is America/New_York, it's calculated as -04:00, even though in December it would actually be -05:00). This is complicated by the fact that the site timezone and browser timezone can be different.