Authoritative data source & methodology

Primary source: IANA Time Zone Database (tzdb), maintained by the Internet Assigned Numbers Authority (IANA). Latest release and historical DST rules are applied by your browser’s implementation of Intl.DateTimeFormat. See the official project page and releases.

“Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.”

The formula explained

\[ t_{\text{target}} \;=\; t_{\text{source}} \;+\; \Delta_{\text{UTC}}(\text{target},\, t) \;-\; \Delta_{\text{UTC}}(\text{source},\, t) \] where \(\Delta_{\text{UTC}}(z, t)\) is the UTC offset (including DST) for time zone \(z\) at instant \(t\).

Glossary of variables

  • \(t_{\text{source}}\): the input wall-clock date and time in the source zone.
  • \(t_{\text{target}}\): the resulting wall-clock date and time in the target zone.
  • \(\Delta_{\text{UTC}}(z, t)\): total offset from UTC (e.g., +02:00) for zone z at instant t, accounting for DST.

How it works: a step-by-step example

Suppose it’s 2025-10-27 09:00 in Europe/Rome. Add targets UTC and America/New_York.

  1. Find the UTC instant corresponding to 2025-10-27 09:00 Europe/Rome using tzdb rules.
  2. Apply each target’s offset at that instant. If New York observes DST then, its offset is \(-04{:}00\); Rome is \(+01{:}00\) or \(+02{:}00\) depending on DST.
  3. Compute \(t_{\text{target}}\) for each zone and label yesterday/today/tomorrow relative to the source date.

FAQ

Does this handle daylight saving time (DST)?

Yes. Offsets are read from your browser’s tzdb implementation, which includes historical and future DST transitions.

Why do I see different abbreviations (PDT, PST, CET)?

Abbreviations vary by date and region. We show a short time-zone name from the browser (e.g., GMT-7 or PDT) for clarity.

What if my browser lacks a specific time zone?

Older browsers may have a reduced tz set. You can still type or select common zones; unsupported ones are hidden.

Is this precise around DST change hours?

Ambiguous or skipped times around transitions are resolved using the platform rules. The calculator runs a two-pass alignment to improve accuracy.

Can I export or print?

Use your browser’s print function. The page is optimized for clean printing.

What about leap seconds?

Consumer systems typically ignore leap seconds; conversions use civil time (UTC) as provided by the OS/browser.

Tool developed by Ugo Candido. Content verified by the CalcDomain Editorial Board.
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