Local Daylight Time (LDT) is a smart clock that knows what time it should be. The LDT smart clock adjusts for the position of the sun at every location and date. Conventional clocks use the Standard Time system created by railroads in 1847 and then endlessly redefined by governments around the world. The result is a hodgepodge of arbitrary boundaries and clock changes based on an artificial sun.

Local Daylight Time has no time zone boundaries, and it ends the hassle and jet lag of changing clocks twice a year. Local Daylight Time varies continuously with location and changes gradually, never jumping to the next hour. As the seasons change, small automatic adjustments of up to one minute per day reduce sunrise variation year round, extending summer evening daylight without the disruption of Daylight Saving Time changes.

Local Daylight Time is ideal for local purposes, such as getting up and going to work. The consistent sunrise time keeps circadian rhythms natural and undisturbed, with proven health benefits. LDT works in harmony with Universal Coordinated Time (UTC) which is well suited for global time over large distances. UTC works for international conference calls, satellite broadcast times and airline schedules, while Local Daylight Time keeps communities perfectly synchronized with the sun, every day, every where.

Local Daylight Time varies with the date, like the sun. Change the date here to see this effect. Latitude (north or south of the equator) also affects Local Daylight Time. The length of the day changes with latitude, from constant at the equator to extremes of midnight sun near the poles. Local Daylight Time shifts smoothly, adjusting sunrise time proportionally and adding evening daylight hours without over extending. At extreme latitudes LDT follows mean solar time, so clocks track the sun even when it does not rise or set.

Local Daylight Time (LDT) FAQ

What is it? – A new smart clock that knows what time it should be. LDT minimizes seasonal sunrise time variation, eliminates the need for time zones and ends changing clocks twice a year.

Why is sunrise important? – It’s healthy. People have a biological clock that is set by exposure to daylight. Keeping a natural schedule reduces depression and sleep disorders.

Aren’t time zones necessary? – No. Time zones are a coarse division of the earth’s rotation, with one hour jumps at arbitrary boundaries. Local time changes continuously in every direction, not just east and west. The change is small, three to six seconds per mile is typical.

No spring forward, fall back? – Actually yes, but without the jet lag. Local Daylight Time adjusts to the changing sunrise time, gradually, by about one minute per day or less. You won’t notice.

So time changes every day and every where. Isn’t that confusing? – LDT is simpler than the Standard Time system. With a smart clock, all that matters is the location and the date. There is no need to keep track of complex boundaries or government rules about daylight saving clock changes.

Smart clock, do I need to buy one? – No, Local Daylight time is free, from the Internet or your cell phone. You might need a new wall clock or a smart alarm clock, but there’s an app for that.

Why hasn’t this been done before? – Technology wasn’t ready for it. Standard time was created by the railroads in 1847. Daylight Saving time began in 1918. Now with computers, cell phones, GPS and the Internet, a better kind of clock time is both possible and practical.

I drive 30 miles to work. How will I get there on time? – You can set your LDT clock to match the one your boss uses. It will only be a few minutes different, unlike many commutes that cross time zones.

How do I schedule a conference call? – When long distances are involved, the best way is by using UTC, Universal Coordinated Time. That is what Standard Time is based on, but without the time zone confusion. You can check others local time to see when they are awake, but make the call by UTC so there is no confusion. UTC is also good for flight schedules.

Doesn’t LDT replace UTC? – No. Two time systems are needed. Local Daylight Time is for local purposes, UTC is for global purposes. Standard Time is just UTC with offsets for time zones and Daylight Saving. Those offsets are no longer needed except for some government regulations.

So sunrise is at the same time every day? – Not exactly. Because the longest day gets longer at high latitudes, a fixed sunrise time isn’t possible. Above the arctic circle, sunrise may not happen for months. In London, a constant sunrise time would push sunset to be after 11:30 pm in the summer. So Local Daylight Time reduces sunrise variation but lets it vary more at higher latitudes.

So how much does sunrise time vary? – At the equator LDT makes sunrise exactly the same every day, adjusting by ten to twenty minutes during the year. In San Francisco (38° N), sunrise varies from average by less than half an hour, compared to more than one hour with Standard Time. The effect is similar to Daylight Saving time but the irritation of changing the clocks and the randomness of time zone placement are gone.

Calculation of Local Daylight Time

Local Daylight Time is a new system of civil time designed to upgrade current time systems, eliminating the discontinuities of time zones and seasonal clock changes while retaining the benefits of Daylight Saving with reduced variation in sunrise time. The time is calculated for each unique location and date as explained below.

The basis of Local Daylight Time (LDT) is Coordinated Universal Time (UTC1). The LDT for a given location and date is found using an offset from UTC summed with the actual UTC. All times are in units of hours.

The LDT offset has several terms. First, a constant offset of 0.5 hours is added, half of the Daylight Saving or Summer Time offset used by many systems of Standard Time. This constant reduces the average difference from existing systems. The added half hour shifts the mean sunrise (and sunset) at equinox to be 6:30 instead of 6:00 (neglecting atmospheric refraction). Social preference for a later sunrise time is evident in both the use of Daylight Saving time and the tendency to skew time zones to the west from their original positions.

Next, an offset component based on longitude is included as used in mean solar time. Dividing the longitude in degrees by 15 (°/h) produces an offset in hours that is proportional to the ratio of 24 hours to a full rotation of 360 degrees. A continuous offset is produced, eliminating time zone discontinuities. The longitude offset polarity is positive for locations west of zero degrees, up to a maximum of less than 24 hours. Longitude coordinates are expressed as

The geographic coordinates and date are used to calculate the local apparent sunrise time. This is a complex calculation but it is available from NOAA, both in Excel and Javascript. The PHP language includes a built in sunrisefunction2. The use of sunrise time introduces a latitude term that follows the illumination map and thus improves tracking between clock time and daylight.

A Local Daylight Time offset term reduces seasonal sunrise time variation. However, since the day length at higher latitudes may be zero or 24 hours, an adjustment to this offset component is required. A suitable function can be derived from the equation of the length of the day. Adding the difference between the ideal mean sunrise (6am) and the actual sunrise would cause sunrise to occur at the same time every day. Because sunrise may not occur at all above the polar circles, an adjustment function reduces this offset term at higher latitudes, allowing full correction (adj=1) only at the equator, and reducing sunrise adjustment to zero at high latitudes.

If the adjustment becomes negative, the value is kept at zero so that no sunrise time correction is applied.

Finally, to avoid a daily discontinuity due to change in the offset, the sunrise offset is calculated both for the current date and the next day. The difference is then linearly interpolated and spread over the current day so that the offset changes continuously.

The complete equation for Local Daylight Time is this:

It should be noted that including a sunrise term causes the length of each day to vary, as the sunrise time varies seasonally. Sunrise time varies with the date at all latitudes, but the daily change of the LDT Offset is small. The interpolation term varies continuously, so the LDT clock runs slightly fast or slow as needed to achieve the next offset value. This achieves the Daylight Saving effect without the one hour discontinuities.

1UTC includes the use of leap second to correct for the changing rate of the earth's rotation. While leap seconds introduce discontinuities that are contrary to the continuous character of LDT, this flaw is accepted as a necessary compromise given the need for an agreed basis time reference. The infrequent one second shift is imperceptible relative to the daily change of the natural day. The leap second discontinuity may be avoided as done by Google

2PHP and other sunrise functions include a term for atmospheric refraction to account for the difference between the 90 degree hour angle and the hour angle of the apparent sunrise. For simplicity, LDT adopts a constant value of 90.833 degrees for the sunrise zenith. Atmospheric refraction also affects the latitude offset adjustment function, which is based on the ratio of the tangent of latitude to the tangent of the polar circles.

NB Constants - Atmospheric refraction varies with conditions but 50' is widely used for sunrise and is a good approximation. The obliquity of the ecliptic varies over time but this has an effect of less than one offset minute over the next 1,000 years. 6:30 for the target sunrise time is somewhat arbitrary, but fits well with present time systems.