[openstreetmap/openstreetmap-website] Timezone mismatch between heatmap data and tooltips (Issue #6378)

[AJ Ashton]

ajashton created an issue (openstreetmap/openstreetmap-website#6378)

### URL

_No response_

### How to reproduce the issue?

On the contributions heatmap shown on user profile pages, the rows and columns appear to be aligned to UTC. However the date shown in the hover tooltip is localized to the browser's time zone. This can lead to a mismatch between the data being visualized and the date being displayed.

For example, today is Thursday September 4, but when I hover over the Thursday row for this week on my heatmap, the tooltip says "September 3" when I view it from UTC-4:00.

<img width="2113" height="301" alt="Image" src="https://github.com/user-attachments/assets/f1203967-4da8-4037-8aba-d457b3d92388" />

I think the tooltip dates should use UTC to match the grid. Alternatively the grid could be aligned to the browser's local time zone, but I assume that would be a much more complicated solution.

### Screenshot(s) or anything else?

_No response_

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