Wall time is not always an instant
A date such as “02:30 on 29 March” is only a wall-clock label until a region is supplied. Most of the year, an IANA zone maps that label to one UTC instant. Near daylight-saving transitions, it can map to none or two.
The 02:xx hour never occurs.
The 01:xx hour occurs twice.
Missing time during spring forward
If a region jumps from 01:59 to 03:00, every 02:xx wall time is nonexistent. Silently moving the input forward can hide scheduling defects, so this tool reports the gap.
Repeated time during fall back
When clocks move backward, the same wall time occurs under two offsets. The results show both possible UTC instants so an event can be disambiguated explicitly.
Example: a New York meeting
New York changes offset seasonally, while UTC does not. A recurring 09:00 New York meeting therefore moves by one hour in UTC when daylight-saving rules change. A fixed UTC schedule behaves differently from a fixed local schedule.
America/New_York carries historical and future transition rules. UTC-05:00 does not know when the region observes daylight saving time.Browser data and future rule changes
Results depend on the IANA time-zone data supplied by the visitor's browser. Governments can change future rules with little notice, so recheck important future schedules near the event date.
DST questions
Does UTC observe daylight saving time?
No. UTC does not move its clock seasonally. Regional zones define their offsets relative to UTC.
Why is an abbreviation such as CST not accepted?
Abbreviations are ambiguous. CST can refer to different regions and offsets. IANA names such as America/Chicago identify the intended rule set.