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What does this converter do?
Type any 5-digit ZIP code and the calculator returns the city, state, county, coordinates, IANA timezone, and the live UTC offset with local time. For example, ZIP 00601 in Adjuntas, Puerto Rico (latitude 18.18004, longitude -66.75218) maps to America/Puerto_Rico at a fixed UTC-4, so 16:00 UTC equals 12:00 PM local, while ZIP 33101 in Miami maps to America/New_York and shows 11:00 AM EST at the same UTC moment in winter.
How the ZIP code timezone lookup works
The source table stores one row per active ZIP code with nine fields: ZIP, preferred city name, state code, county name, county FIPS code, latitude, longitude, IANA timezone, and a population estimate. When you enter a ZIP, the calculator reads the matching row and displays the city, state, county, coordinates, and timezone together with the offset in force right now.
The time math is one addition: local time equals UTC time plus the zone offset. The offset comes from the IANA rules for the date you check, so a New York ZIP returns UTC-5 in January (EST) and UTC-4 in July (EDT), while Phoenix returns UTC-7 in both months because most of Arizona does not observe daylight saving.
Where a 5-digit code carries more than one record, for example a shared federal ZIP, the calculator lists each match with its state and county so you can confirm you have the right one before reading the time.
Why the zone does not always match the map
Timezone lines in the US mostly follow county borders rather than meridians. That is why Gary, Indiana runs on Central time roughly 140 miles from Eastern-time Indianapolis, and why western Kentucky and Tennessee split down the middle even though their longitudes look identical to neighboring Eastern states.
Several US regions never move their clocks: most of Arizona (UTC-7 year round), Hawaii (UTC-10), Puerto Rico and the US Virgin Islands (UTC-4), Guam and the Northern Mariana Islands (UTC+10), and American Samoa (UTC-11). The tzdata zone attached to each ZIP encodes these choices, so the calculator applies them without extra input.
The coordinates stored with a ZIP mark the center of its delivery area, which can cover hundreds of rural square miles. If a result seems off for a remote address, compare the county field on the result card with the address's own county, since the zone assignment follows the county table.
How this calculator works
Formula
The calculator looks up your 5-digit ZIP code in a USPS ZIP table where every row carries a latitude, longitude, county, and IANA timezone name. It reports the zone plus its UTC offset for the selected date, computed as local time = UTC time + offset, with the offset rising by one hour during daylight saving for zones that observe it.
Worked example
ZIP 00601 resolves to Adjuntas, Puerto Rico (FIPS 72001, latitude 18.18004, longitude -66.75218) in the America/Puerto_Rico zone. That zone keeps Atlantic Standard Time at UTC-4 all year with no daylight shift, so at 16:00 UTC the local time is 16 - 4 = 12:00 PM AST on any date. For contrast, ZIP 33101 in Miami sits in America/New_York: in December it reads 16 - 5 = 11:00 AM EST, and in July it reads 16 - 4 = 12:00 PM EDT.
Assumptions to verify
- Each ZIP code resolves to one primary timezone taken from the center coordinates of its delivery area; ZIP codes that straddle a boundary follow the side holding most of the population.
- Daylight saving offsets follow the current IANA tzdata rules, with US daylight saving running from the second Sunday in March to the first Sunday in November for participating zones.
- The ZIP table reflects the active USPS ZIP code list at the time it was built, so very new, retired, or PO-box-only records may not return a result.
Frequently asked questions
How many timezones cover US ZIP codes?
The 50 states, DC, and the territories span 10 base zones: Eastern, Central, Mountain (with a separate no-daylight-saving Arizona zone), Pacific, Alaska, Hawaii-Aleutian, Atlantic, Chamorro, and Samoa time. The IANA names behind them include America/New_York, America/Chicago, America/Denver, America/Phoenix, America/Los_Angeles, America/Anchorage, Pacific/Honolulu, America/Puerto_Rico, Pacific/Guam, and Pacific/Pago_Pago, plus a few county-level variants in Indiana, Kentucky, and North Dakota.
Does the result change with daylight saving time?
Yes. Enter a date and the calculator returns the offset in force on that date, so New York ZIPs show UTC-5 (EST) in January and UTC-4 (EDT) in July. Zones without daylight saving, including Arizona, Hawaii, Puerto Rico, Guam, and American Samoa, keep a constant offset every day of the year.
Why does my ZIP code show a different city or county than I expected?
USPS assigns each ZIP code to a single delivery processing point, and the primary city name in the table is that point's city, not necessarily your mailing address city. The county and coordinates refer to the center of the delivery area, which can sit miles from any given address inside the ZIP.
Cite this tool
BoringToolsKit. “ZIP Code Timezone Calculator.” boringtoolskit.com/zip-timezone-calculator/. Free to reference in articles, syllabi, and answer posts with a link.
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