Timezone doubt

[kepler]

Hi

I have a doubt that someone might help to clear.

In my calculations I often correct the times to UTC, using the time offset difference.
The problem is that I need to know if at a particular time there were tz rules or not, because if so the correction is not the general longitude given, but the longitude of the location itself.

Is there a way to know this?
Example: a date of 1823 in America/Chicago in a nearby city. I need to know if at that time the TZ offset was in use or not. If yes, I use that value - Chicago as reference. If not, I use the nearby city longitude.

Kind regards,

Kepler

[requinix]

By "timezone offset" I assume you meant whether DST is in effect? date() and DateTime::format() both have a 'I' specifier indicating whether DST was in effect for the timestamp.