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I've seen a product where they used UTC to store opening hours. They had to rewrite all dates via a script twice a year.

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I don't know the exact use case, but it sounds weird. If UTC is the single source of truth, then when you display the data and want to change some business assumptions, you only need to change the application logic. The underlying data remains unchanged.

One scenario I can imagine is when you want to store the results of business calculations in the database. In that case, once the algorithm changes, you may also need to update the stored data. This may be necessary for performance reasons.

In all other cases, calculating the result on the fly solves the problem and does not require a database update when the business rules change.

Why would the dates need rewriting?

I suspect they meant more generally “the date&time field”, but a store in Boston that opens at 7:30 AM and closes at 7:30 PM Eastern time (ET) closes on different UTC date than it opens when ET is EST but opens and closes on the same date when ET is EDT, so it’s plausible that the dates actually needed to be updated.

Daylight saving changed the UTC time, because opening hours were always in local time.

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