Time Zones and ZIP Codes

## ZIP Codes and Time

The United States spans six time zones (including Alaska and Hawaii), and ZIP codes do not align perfectly with time zone boundaries. Most ZIPs fall entirely within one time zone, but border-area ZIPs can span two.

## US Time Zones

| Zone | UTC Offset | States |
|------|-----------|--------|
| Eastern (ET) | UTC-5 | 22 states + DC |
| Central (CT) | UTC-6 | 20 states |
| Mountain (MT) | UTC-7 | 8 states |
| Pacific (PT) | UTC-8 | 5 states |
| Alaska (AKT) | UTC-9 | Alaska |
| Hawaii (HST) | UTC-10 | Hawaii |

## Split-Zone States

Several states observe two time zones:

- **Indiana** — Most is Eastern, but 12 western counties are Central
- **Tennessee** — East Tennessee is Eastern, West Tennessee is Central
- **Kentucky** — Eastern half is Eastern, Western half is Central
- **Nebraska** — Most is Central, western panhandle is Mountain
- **North Dakota** — Most is Central, southwest corner is Mountain
- **Oregon** — Most is Pacific, Malheur County is Mountain

## The Arizona Exception

Arizona does not observe Daylight Saving Time (DST), except for the Navajo Nation, which does. This means:

- In winter: Arizona = Mountain Standard Time = UTC-7
- In summer: Arizona = Pacific Daylight Time equivalent = UTC-7
(but California and Nevada shift to UTC-7 for PDT)

## Practical Implications

Time zone mapping by ZIP is important for:

- **Call centers** — Compliance with state telemarketing laws (no calls before 8am or after 9pm local)
- **TV scheduling** — Broadcast times differ by zone
- **Logistics** — Delivery time windows and cutoff times
- **SaaS applications** — Displaying correct local time for users

## Mapping ZIP to Time Zone

Since time zone boundaries follow county lines (mostly), and ZIP codes can span counties, the mapping is not always 1:1. The most reliable approach is:

1. Use the ZIP centroid (latitude/longitude)
2. Overlay with a time zone shapefile (IANA tz database)
3. Handle edge cases manually (Navajo Nation, split counties)