I have another post on this issue, but just saw this.

I have Sherlocked that a c++ function that convert time to epoch set the DST parameter to inactive. I do not have a confirmation that this is the case though, just a find in the driver (lx200 driver).

This suggests (for me at least) that the time from the mount is correct when DST is inactive.

My suggestion is try to use Julian Day instead, in this case: Convert mount time to julian->Add UTC offset (in Julian hours)->Convert back to gregorian. Need to be tested if DST also is an issue with Julian Day though.

Mount is Meade EXT90

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