Time and Time Conversions (UTC, LMT, Time Zones, Date Line)

Time systems and time conversions

For aviation, precise and uniform time measurement is essential:

• Flight plans, METARs, NOTAMs, ATC clearances are published in UTC.
• Sunrise/sunset determines VFR flight possibilities.
• International flights cross time zones and possibly the dateline.

UTC — Coordinated Universal Time

UTC is the international time standard of aviation.

Properties:

• Identical to GMT (Greenwich Mean Time) — but UTC is defined by atomic clocks (TAI), GMT astronomically.
• Leap seconds keep UTC < 0.9 s from astronomical UT1.
• No daylight saving — UTC is constant.

Aviation notation: "Z" or "Zulu Time".

Format: HH

Significance:

• METAR: "EDDF 271320Z" = Frankfurt METAR of the 27th, 13
  UTC.
• TAF, NOTAM, flight-plan times: all in UTC.

Local Mean Time (LMT) and Local Time

Local Mean Time (LMT) is the local mean solar time at a place:

• Noon sun in the south of the observer → 12
  LMT.
• LMT varies continuously with geographic longitude — 4 minutes per degree (15° = 1 hour).

But: LMT is impractical for daily life because it shifts every 1° west by 4 minutes. Hence: time zones.

Time zones

Concept: Earth is divided into 24 zones of 15° longitude each. Inside a zone a uniform "zone time" applies.

Standard time zones (examples):

• UTC (0°): UK, Portugal, west Africa.
• UTC+1 (15° E): Central Europe (Berlin, Paris, Rome) — CET.
• UTC+2 (30° E): Eastern Europe (Helsinki, Athens, Cairo) — CEST (summer DST in CET).
• UTC−5: US East Coast (New York) — EST.
• UTC−8: US West Coast (Los Angeles) — PST.
• UTC+9: Japan.

Central Europe conversion:

Daylight Saving Time (DST)

Many countries advance their local time by 1 hour in summer:

• In Europe: last Sunday in March → CEST = UTC+2.
• End: last Sunday in October → CET = UTC+1.
• USA: second Sunday in March to first Sunday in November.
• Tropical countries (e.g. India, Japan, Singapore): usually no DST.

Important: UTC stays constant — only local time changes.

Time conversion — practice

Example 1: Frankfurt METAR at 14

• UTC + 1 (CET) = 15
  Berlin local.

Example 2: ETA in New York 19

• 19
  − 4 = 15
  EDT.

Example 3: Planned sunset at destination 20

• LMT vs UTC: 12° E / 15°/h = +48 min (east of Greenwich means LMT ahead of UTC).
• 20
  LMT − 48 min = 19
  UTC.

International Date Line (IDL)

Concept: a line approximately along 180° longitude at which the date changes.

Properties:

• Passes through the Pacific, curving around island groups.
• East of the line: date is one day earlier.
• West of the line: date is one day later.

Example:

• Flight Tokyo (UTC+9) → Los Angeles (UTC−8) across Pacific:
  • 12 hours flight time
  • Departure Tokyo 22
    local on the 27th.
  • Arrival LA: 22
    + 12 h = 10
    UTC on the 28th.
  • LA local: 10
    − 8 h = 02
    on the 28th ← same day due to IDL!

For PPL usually not relevant, but important for trans-Pacific flights.

Sunrise and sunset

Calculation:

• Astronomical almanacs and online calculators (NOAA, USNO, timeanddate.com) give sunrise/sunset times by date and location.
• Important for PPL:
  • Begin Civil Morning Twilight (BCMT) / End Civil Evening Twilight (ECET) — sun 6° below horizon; standard definition of "night" in aviation.
  • Nautical twilight — sun 12° below horizon.
  • Astronomical twilight — sun 18° below horizon.

Definition of "night" in EU aviation (SERA, FCL):

• Time between end of evening civil twilight and beginning of morning civil twilight.
• In practice: ~30 min after sunset to ~30 min before sunrise (rough rule).

Sunrise/sunset in AIP:

• The AIP GEN contains sunrise/sunset tables.
• Also integrated in many EFB apps.

VFR implications

Night flying:

• VFR night is permitted only with Night Rating (FCL.810).
• Look up times in the AIP before flight.
• Rule of thumb in mid latitudes:
  • Summer: sun up very long → little night-flight pressure.
  • Winter: sunset early (16
    local in Berlin on 21 December) → daylight short.

Flight-plan times:

• All flight-plan times in UTC!
• Confusing local with UTC is a frequent error.