Datetime DataForm control select the previous day on selection

[ntsekouras]

Discovered here: #81337 (comment)

When we use datetime DataForm control and select a day, the previous one is actually selected. We should also investigate if this is the case for the rest date/time DataForm controls.

Testing instructions

In storybook: https://wordpress.github.io/gutenberg/?path=/story/dataviews-fieldtypes--date-time-component select the item and try then try to change the date from the UI. You can observe that the previous day is selected.

Screen.Recording.2026-08-07.at.5.06.41.PM.mov

[Mustafabharmal]

I'm working on this.

Root cause: the calendar reads and reports the plain Dates it's given in its own timezone frame, the timeZone prop, or the browser's timezone when that prop is absent. Both controls pass timeZone={ getSettings().timezone.string || undefined } alongside plain Dates, and timezone.string is empty whenever a site uses a manual UTC offset instead of a named zone (a default install, and the Storybook defaults). So the calendar falls back to the browser's timezone while the values stay anchored to the site's, and dateI18n( 'Y-m-d', newDate ) re-projects the clicked day's midnight into the site timezone, landing on the previous day whenever the browser is ahead of the site. That's why it reproduces in Storybook from UTC+3 but not from UTC.

On the second part of the issue: yes, date is affected by the same root cause, in the opposite direction. It runs a tz-less yyyy-MM-dd value through getDate(), so on positive-offset sites the stored day renders highlighted a day early, and the Today preset doesn't round-trip. The between range calendar shares it.

[oandregal]

An interesting fact is that the calendar in filter keeps the day (but the input does have the day before):

Screen.Recording.2026-08-10.at.11.45.41.mov