Calculate local mean time from standard time and longitude, reverse the conversion, or infer longitude from the four-minutes-per-degree offset.

Use standard time with daylight saving removed. Mean solar time excludes the equation of time. Enter the actual zone reference meridian; longitude does not determine legal time zones.

24-hour HH:MM or HH:MM:SS. Remove daylight saving before entry.

Use 0° for the Greenwich reference. A standard offset of +1 h corresponds to 15° E.

LMT Formula

The LMT calculator converts between standard-zone clock time and local mean solar time using the location and reference meridian. In signed east-positive degrees, LMT = standard time + 4 minutes × (location longitude − standard reference meridian). The Greenwich examples below use a reference meridian of 0°.

LMT = GMT + Δ T

Where:

  • LMT = local mean time at the location
  • GMT = Greenwich Mean Time used as the reference time
  • ΔT = time difference caused by longitude

Longitude to Time Conversion

Earth rotates 360 degrees in 24 hours, so every 15 degrees of longitude corresponds to 1 hour of time difference. East longitudes are ahead of Greenwich, and west longitudes are behind Greenwich.

Δ T = (Longitude) / (15)
1^° = 4 minutes

This means you can convert longitude into a time offset before using the calculator:

  • Positive offset for east longitude
  • Negative offset for west longitude
  • Enter longitude and the standard reference meridian; the calculator derives the correction rather than accepting a separate offset input
Longitude Offset from Greenwich
15° E +1 hour
30° E +2 hours
45° W -3 hours
90° E +6 hours
120° W -8 hours

24-Hour Clock Adjustment

After adding the longitude offset, the result should be expressed on a 24-hour clock. If the result is above 24 hours, subtract 24 hours. If it is below 0 hours, add 24 hours until it falls within the normal daily range.

LMT₂₄ = ((GMT + Δ T) bmod 24 + 24) bmod 24

In plain terms:

  • If your answer is 26:00, the local mean time is 02:00
  • If your answer is -0:30, the local mean time is 23:30

How to Calculate LMT

  1. Determine the standard-zone clock time and its reference meridian. Remove daylight saving first.
  2. Enter the location longitude and direction, plus the standard reference meridian and direction.
  3. The calculator converts the signed longitude difference to time at four minutes per degree.
  4. Add the correction to standard time, or subtract it for the reverse conversion.
  5. Wrap the result into the 24-hour range if necessary.

For calculator use, the most common input mistake is the sign of the longitude. East is positive, west is negative.

Examples

Example 1: If the reference time at Greenwich is 07:00 and the location is 90° east, the longitude offset is 6 hours ahead of Greenwich.

Δ T = (90) / (15) = 6 h
LMT = 7 + 6 = 13 h

The local mean time is 13:00.

Example 2: If the reference time at Greenwich is 02:30 and the location is 45° west, the longitude offset is 3 hours behind Greenwich.

Δ T = (- 45) / (15) = - 3 h
LMT = 2.5 - 3 = - 0.5 h = 23:30

The local mean time is 23:30 on the previous reference day after adjusting to the 24-hour clock.

Why Local Mean Time Is Useful

LMT is useful whenever time needs to be tied directly to longitude rather than to a modern legal time zone. It is commonly used in:

  • astronomy and observational timing
  • historical records and old navigation references
  • solar position and daylight studies
  • understanding how mean solar time differs from standard clock time

Important Notes

  • LMT is not the same as civil clock time. Time zones, daylight saving time, and political boundaries can make local clock time different from longitude-based mean time.
  • Use 24-hour clock inputs. Enter HH:MM or HH:MM:SS. Decimal-hour entry is not supported; 7.5 hours corresponds to 07:30.
  • Check the sign of the offset. A west longitude entered as positive will shift the result in the wrong direction.
  • Keep the time reference clear. Mean solar time excludes the equation of time. Greenwich mean solar time is represented by UT1; UTC is an approximation and no UT1−UTC correction is applied.

FAQ

What does LMT stand for?
LMT stands for Local Mean Time.

How do I get the longitude time difference?
Divide the longitude in degrees by 15 and assign a positive sign for east or a negative sign for west.

What if the result is negative?
Add 24 hours until the result falls within the standard 24-hour range.

Can I use minutes and seconds?
Use HH:MM or HH:MM:SS clock notation. Outputs round to the nearest second. Longitude inference uses the nearest signed clock difference within ±12 hours and returns longitude modulo 360°; clock inputs alone do not identify dates.