Amazon Summer Time (Historical)
UTC offset: -03:00 (historical, summer only)
Standard offset: -04:00 (AMT, current year-round)
IANA identifier: America/Manaus
Abbreviation: AMST (no longer active)
DST status: Discontinued
Amazon Summer Time advanced Brazil's northern interior one hour from AMT (UTC-04:00) to UTC-03:00 during the southern hemisphere summer. At -03:00, the Amazon states temporarily aligned with Brasilia Time, eliminating the usual one-hour gap to southeastern Brazil.
The practice was applied inconsistently to Amazon region states over the decades. Some years they participated, other years they didn't. Ultimately Brazil abolished all DST nationwide in 2019 (effective from that year's summer), ending the question permanently. The Amazon states hadn't meaningfully participated for several years before that.
Why It Never Made Sense
The AMT states sit between the equator and about 12S latitude. Manaus at 3.1S gets 12 hours and 7 minutes of daylight at the June solstice and 12 hours and 7 minutes at the December solstice. That's effectively zero seasonal variation. Moving the clock an hour at those latitudes saves no energy, provides no usable extra evening light, and simply disrupts schedules.
The Brazilian government's own studies showed energy savings from DST were concentrated in the southeastern states (Sao Paulo, Rio de Janeiro, Minas Gerais) where summer days are noticeably longer. Applying the same policy to equatorial regions was administrative convenience rather than evidence-based policy.
States Historically Affected
When the Amazon states did participate in DST, the following were included:
- Amazonas (Manaus): Population ~4.2 million
- Roraima (Boa Vista): Population ~630,000
- Rondonia (Porto Velho): Population ~1.8 million
- Mato Grosso (Cuiaba): Population ~3.6 million (sometimes included in AMT zone)
- Mato Grosso do Sul (Campo Grande): Population ~2.8 million (sometimes)
Participation varied by presidential decree. States near the border between AMT and BRT zones sometimes switched which regime they followed.
Impact on the Manaus Free Trade Zone
The Zona Franca de Manaus (500+ companies, 100,000+ workers) benefited from DST alignment with Sao Paulo. During AMST, Manaus and Sao Paulo shared the same clock (-03:00), simplifying logistics, conference calls, and supply chain coordination. Without DST, the one-hour gap persists year-round, requiring Manaus operations to start or end their day offset from southeastern headquarters.
However, the manufacturing workforce generally preferred not having the clock disruption. Factory shift schedules, school start times, and bus routes all had to adjust twice per year during DST participation.
Brazil's DST History
Brazil used DST intermittently from 1931 onward:
- 1931-1933: First adoption
- Various periods of use and disuse through the 20th century
- 1985-2019: Most consistent period of annual DST (though coverage varied by state)
- 2019: President Bolsonaro signed decree ending DST, citing studies showing negligible energy savings
The end of DST was popular nationally. Polls showed about 75% of Brazilians preferred not changing clocks.
Climate Context
The equatorial climate of the AMT states means:
- 12 hours daylight year-round (±7 minutes)
- No meaningful "summer" or "winter" in the temperate sense
- Seasons defined by wet (December-May) and dry (June-November)
- Temperature: 26-33C year-round
This climate reality made DST participation purely an administrative/political decision rather than one driven by actual daylight patterns.
Technical Identifiers
- America/Manaus (IANA canonical)
- AMST (Amazon Summer Time, UTC-03:00, historical)
- AMT (current, Amazon Time, UTC-04:00)
- Windows: "SA Western Standard Time"
- Brazil abolished all DST in 2019
Quick Reference
| Attribute | Value |
|---|---|
| Historical summer offset | -03:00 |
| Current UTC offset | -04:00 (permanent) |
| DST status | Abolished (2019 nationwide) |
| IANA zone | America/Manaus |
| Population in zone | ~25 million |
| Largest city | Manaus (~2.2 million) |
| Daylight variation | ~14 minutes total (equatorial) |
| Reason abolished | No energy savings at equatorial latitude |