Offsets can change during the year
New York commonly switches between UTC-5 and UTC-4, while London switches between UTC+0 and UTC+1. Other places do not observe daylight saving time at all. The difference between two cities can therefore change even when neither city changes its geographic location.
Skipped and repeated local times
When clocks move forward, a range of local clock readings may never occur. When clocks move backward, one range occurs twice. A local value such as 01:30 can therefore be invalid or ambiguous on a transition date unless an offset or other disambiguation is supplied.
Use zone names instead of memorized offsets
A named zone lets a conversion library select the rule for the requested date. Writing “New York time” as UTC-5 throughout the year creates a one-hour error during daylight time. Future political changes also make hard-coded assumptions fragile.
Safer scheduling practices
- Store completed events as UTC instants.
- Keep the IANA zone for future or recurring local schedules.
- Include an explicit offset when exchanging a resolved timestamp.
- Test dates near spring and autumn transitions.