[6.x] Avoid calling ->startOfDay() on date range dates

[duncanmcclean]

This pull request avoids calling ->startOfDay() when augmenting date ranges to avoid situations where the user selects one date but it saves another, which may cause the wrong date to be displayed on the frontend.

For example: if you're in Germany and select the 2nd February, it'll save as 2026-02-01 23:00.

Related: #13839 (comment)

[ajnsn]

Hey @duncanmcclean — thanks for this PR. As this issue currently breaks the whole application on our end (all event date ranges are shifted by one day), we’d really appreciate it if this could be merged soon.

An alternative solution would be to set date_format: Y-m-d directly on the field, which seems causing the same issue ☺️ Do you know if this will be addressed by the augmentation change as well?

[duncanmcclean]

An alternative solution would be to set date_format: Y-m-d directly on the field, which seems causing the same issue ☺️ Do you know if this will be addressed by the augmentation change as well?

Even when this PR is merged, you shouldn't save date range dates in Y-m-d as Statamic needs the correct date & time in order to convert from your app's timezone (by default, UTC) into your browser's timezone.