Daylight-Saving Transition Explorer
See when clocks change in a time zone. Find skipped or repeated times and compare each local time with UTC.
Does this local time exist once, twice, or not at all?
Enter an IANA time zone and a local date and time. The explorer lists every valid UTC interpretation and identifies clock gaps or repeated hours. Review the year's transition table, including the local interval affected and offset changes. Half-hour changes and skipped dates are handled as well as one-hour daylight-saving changes.
Make sense of a clock change.
One wall time can mean two different moments—or no moment at all.
Your explanation and report will appear here.
Three simple steps
How to use Daylight-Saving Transition Explorer
Enter a time, recurrence rule or local calendar file.
Choose the relevant date range and inspect the explanation or overlaps.
Download the report and save a project to reopen your inputs.
Common questions
Good to know
Understand the result.
Keep the original.
Is this tool free, and are my files uploaded?
This tool is free with no account required. The work happens in your browser. Your input and files are not sent to a processing server.
What are the limits and details?
Calculations run in a cancellable browser worker. Time-zone offsets use this browser's installed database unless an imported calendar supplies its own VTIMEZONE. Save a local project file to retain your inputs; files are not uploaded and invitations are not sent.
Years 1970–2100. Annual transition scanning samples six-hour intervals and refines a detected offset change to one second. This tool explains clock changes; future rules may change with legislation.