LMT (Local Mean Time) Calculator

Enter the Greenwich Mean Time (GMT) (or UTC) and the longitude + direction into the calculator to convert to LMT (local mean time).

LMT Formula

The LMT calculator determines Local Mean Time, which is the mean solar time at a specific longitude measured relative to Greenwich. The idea is simple: start with Greenwich Mean Time and shift it forward or backward based on how far east or west the location is.

LMT = GMT + \Delta 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.

\Delta T = \frac{\text{Longitude}}{15}

1^\circ = 4 \text{ 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
If you already know the offset in hours, minutes, or seconds, you can enter that directly

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.

LMT_{24} = ((GMT + \Delta 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

Determine the Greenwich Mean Time.
Find the location’s longitude or enter the longitude time difference directly.
Convert longitude to time if needed.
Add the offset to GMT.
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.

\Delta T = \frac{90}{15} = 6 \text{ h}

LMT = 7 + 6 = 13 \text{ 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.

\Delta T = \frac{-45}{15} = -3 \text{ h}

LMT = 2.5 - 3 = -0.5 \text{ h} = 23{:}30

The local mean time is 23:30 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 decimal hours carefully. For example, 7.5 hours means 7 hours 30 minutes, not 7 hours 50 minutes.
Check the sign of the offset. A west longitude entered as positive will shift the result in the wrong direction.
Keep units consistent. If one value is in hours and another is in minutes or seconds, convert them properly before combining them.

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?
Yes. The calculator accepts hours, minutes, and seconds, so you can work with either decimal time or standard time notation.

LMT Calculator