Why are timezone abbreviations ambiguous?

EST is both US Eastern Standard Time (UTC-5) and Australian Eastern Standard Time (UTC+10). CST is US Central, China Standard, and Cuba Standard Time. IANA names (America/New_York, Asia/Shanghai) are unambiguous. Always use IANA names for storage and computation; use abbreviations only for display.

How do I handle DST transitions?

Spring forward: 2 AM becomes 3 AM — times between 2 and 3 AM do not exist. Fall back: 2 AM becomes 1 AM — times between 1 and 2 AM occur twice. For scheduling, use UTC to avoid ambiguity. For display, a good library will show which occurrence (before or after the transition) you mean. Never schedule events at 2:30 AM — it may not exist or may be ambiguous.

TASK

Convert between timezones correctly

Timezone conversion is tricky due to DST, abbreviations, and political changes. Learn to use IANA timezone names and the Instant/ZonedDateTime pattern.

1

Best practices: (1) Store timestamps in UTC. (2) Use IANA timezone names (America/New_York, not EST/EDT — abbreviations are ambiguous). (3) Convert to local time only for display. (4) Use a library (date-fns-tz, Luxon, js-joda) that supports IANA timezones. (5) Never use Date for timezone math — JavaScript Date is broken for timezones. (6) Be aware of DST transitions (2 AM becomes 3 AM in spring, 2 AM repeats in fall).
2

Timezone Converter →
Convert a timestamp from one timezone to another. Specify the source and target IANA timezone names (e.g., America/New_York to Asia/Tokyo). The tool handles DST transitions correctly. Always use IANA names, not abbreviations (EST could be US Eastern or Australian Eastern).
3

Timestamp Converter →
Get the current UTC timestamp. Store this in databases, APIs, and logs. UTC is universal — no DST, no political changes. Convert to local time only at the presentation layer. Use ISO 8601 format (2024-01-15T10:30:00Z) for storage and exchange.
4

Date Calculator →
Calculate date differences across timezones. Be careful: "add 1 day" in UTC may cross a DST boundary in local time, resulting in 23 or 25 hours. Use date-based arithmetic (add days, not hours) when the intent is calendar-based, and time-based arithmetic (add hours) when the intent is duration-based.

Frequently asked questions

Why are timezone abbreviations ambiguous?: EST is both US Eastern Standard Time (UTC-5) and Australian Eastern Standard Time (UTC+10). CST is US Central, China Standard, and Cuba Standard Time. IANA names (America/New_York, Asia/Shanghai) are unambiguous. Always use IANA names for storage and computation; use abbreviations only for display.
How do I handle DST transitions?: Spring forward: 2 AM becomes 3 AM — times between 2 and 3 AM do not exist. Fall back: 2 AM becomes 1 AM — times between 1 and 2 AM occur twice. For scheduling, use UTC to avoid ambiguity. For display, a good library will show which occurrence (before or after the transition) you mean. Never schedule events at 2:30 AM — it may not exist or may be ambiguous.