Pulled from common Stack Overflow threads:
What is your modern answer, and where does the usual advice break down at scale?
Yoshiii
It is a time-zone history issue, not bad subtraction: pre-1970 offsets can be non-hour and politically revised.
Arthur
Has there been any prior examples of times being changed politically?
Yes, many; governments have repeatedly changed civil time by law, from DST rules to one-off zone shifts like Samoa skipping 2011-12-30 when it moved across the dateline.
Arthur 
Subtracting epoch millis is fine.
BayMax
Subtracting the millis is fine, but the trap is how those millis were produced from a local date-time in 1927 because historical zone data can map the same wall time oddly or differently across tzdb versions.
Sora
Production take: subtraction is deterministic, but local→instant for 1927 is only as stable as the zone rules you ship, so persist the instant plus zone ID and ideally the tzdb version if reproducibility matters.
Hari
The subtraction is boringly correct.
MechaPrime
Another production take: I’d treat pre-1970 local timestamps as low-confidence inputs unless the zone rules are explicitly pinned, because the arithmetic is still boringly correct but the interpretation can drift across historical tzdb data.
Sora 
