Converting Between Mission Elapsed Time (MET)
UTC, Local, and Epoch Time

There are several methods of timekeeping for a Shuttle Mission. Unfortunately, the most natural one, the one you are most familliar with and the time you keep at your location, is not the most commonly used. This section describes the four methods of concern to you, and how to convert between them.

Local Time

Local time is simply the time at your location. It is the time you are used to, and the one that you will need to translate the contact dates and times into. You will need to inform the media, students, parents, school officials, and other members of your team about events in local time. But since the timeline events are scheduled in Mission Elapsed Time, and UTC Time, you will need to know how to convert between all three time keeping standards.

A variant of local time is called Military Time. Conversion between UTC and Local Time is most easily performed if local time is converted to Military Time before it is converted to UTC. When converting from UTC to Local Time, the result will be the local time expressed in Military Time. Military time uses a 24-hour time standard, begins each day at local midnight. Local midnight is called 0000 hours. 1 AM is calledi 0100i (pronounced "Oh-One-Hundred Hours"). Local Noontime is 1200 hours (pronounced "Twelve-Hundred Hours", 1:35 PM is 1335, and is pronounced "Thirteen-Thirty-Five Hours, etc.

Mission Elapsed Time (MET)

As described in Chapter 5, there are many variables which can cause the launch time to slip, and therefore affect the actual date and time that an event will occur during a mission. To reduce the complexity associated with this, NASA bases all of its times from the actual time of lift-off. This time is known as Mission Elapsed Time, or MET. When compared to the lift-off time, all events on board the Shuttle happen at a fixed time. Also, due to orbital mechanics, for any given time following lift-off, the Shuttle will be over the same point on earth, assuming there are no unplanned changes to the Shuttle's orbit. The issues surrounding flight event planning are described in Chapter 5, Mission Planning Basics. SAREX contacts are Earth-Fixed Timeline Events, based on the fact that the elapsed time it takes the Shuttle to arrive over any given location on the Earth is not changed by a launch delay.

METs are stated in days, hours, minutes, and if necessary, seconds after lift-off. Your scheduled contact will be planned to occur at a specific time, MET. For example, if your contact is scheduled to occur at 4 days, 10 hours, 15 minutes, and 35 seconds after lift-off, it will be stated as 4/10:15:35. MET starts with 00/00:00 at T-0, or lift-off, so the first day of the flight is MET day 0. This should not be confused with the flight day. Launch day is flight day 1, and MET day 0.

If you listen in on NASA Air-to-Ground Shuttle Audio retransmission through a amateur repeater, one of the HF band rebroadcasts, or NASA Televison (NTV), You will often hear the Flight Controllers tell the Crewmemebers something like "Perform step 3 of the procedure at four-ten-fourty-five" or you might hear "Perform step three at ten-fourty-five". You might look at your watch and see that it is actually 4:34 in the afternoon. You might then ask yourself, "Its really 16:34". Does this mean they will wait until 10:15 tonight?".

The answer is that the crew and ground controllers are using Mission Elapsed Time. Everything the crew does from lift-off is scheduled in MET. That is the clock the crew keeps on board the shuttle. In this example, if the shuttle launched that morning at 6:00 AM your time, then it is actually 10:34 on board the shuttle. The onboard or MET clock started with 00/00:00:00 at 06:00 your time, and you must subtract this 6 hour offset from your time to arrive at MET.

UTC - Coordinated Universal Time

UTC time is the time standard employed by scientists and engineers, as well as anyone doing business on a global basis. It is also typically the time kept by amateur radio operators when logging contacts or setting contact schedules. This is because UTC time is the same all over the world. It is a 24-hour time schedule, without such notation as AM or PM like Military Time. Military time, which also uses a 24-hour time standard, begins each day at local midnight. Local midnight is called 0000 hours. 1 AM is called 0100i (pronounced "Oh-One-Hundred Hours"). Local Noontime is 1200 hours (pronounced "Twelve-Hundred Hours", 1:35 PM is 1335, and is pronounced "Thirteen-Thirty-Five Hours, etc. Coordinated Universal Time follows a similar format, but the local start time of each day depends on a person's location on the Earth. UTC, (also called Greenwich Mean Time (GMT), or ZULU Time), uses the Prime Meridian at the Royal Observatory in Greenwich, England as its starting point. When it is midnight in at the Royal Obersvatory in Greenwich, England, it is the beginning of a new day worldwide. This time is called 00:00 UTC or in Military Format "Zero Hours, UTC).

This standard is used to simplify timekeeping matters, and brings everyone world-wide who are working together into the same "time zone". This eases comparisons of notes and data. NASA maintains all of its satellite information in UTC format, including the orbital elements (parameters that describe an orbiting objects path through space) for the Shuttle and MIR in UTC. Converting between UTC and your local time is a straight-forward matter. Its is simply adding or subtracting the difference in hours between your time zone and the time zone that Greenwich, England is in. Remember that your time zone or location could be observing Daylight Savings Time (or perhaps isn't during the summer months), and you will need to adjust accordingly. Table 6-1, UTC/Local Time Zone Differences, provides you with the necessary correction factor for the United States.

Table 6-1
UTC/Local Time Zone Differences

Eastern EST = UTC-5 Hours EDT = UTC-4 Hours
Central CST = UTC-6 Hours CDT = UTC-5 Hours
Mountain MST = UTC-7 Hours MDT = UTC-6 Hours
Pacific PST = UTC-8 Hours PDT = UTC-7 Hours

Finally, UTC can also incorporate a date. The format for UTC when the date is included is DDD:HH:MM:SS where:

For example, 19:30:42 UTC on January 8 is 008:19:30:42. 04:55:15 UTC on October 1, 1996 (a leap year) is 275:04:55:15.

Epoch Time

Epoch Time is a UTC time format used for the orbital elements that describe the path through space around the earth, of an orbiting body, such as MIR or the Shuttle. The Epoch Time in orbital element sets is the instant in time (UTC) that the observation was made. The element set describes the motion at that instant. Epoch time is expressed as: YYDDD.fraction where:

To find the current time in Epoch Time, first convert the time to UTC, using the date-inclusive format of UTC. This provides the year and the day and bases your fraction calculation on UTC. Then determine the number of seconds that have passed since Midnight, UTC. Divide this by 86,400 (the number of seconds in a day).

Time Conversion Examples

These examples will show you how to convert between UTC/LOCAL and MET/UTC/LOCAL Times

UTC/LOCAL Time Conversions

Since UTC is a 24-hour based format, you will always need to convert to/from Military Time Format for your local time.


Your school is located in Cincinnati, Ohio. It is June 17, 1994 at 1:15 PM. What is the UTC date and time at your school?

Cincinnati, Ohio is in the Eastern time zone. Daylight savings time begins on the first weekend in April and ends on the last weekend in October. Therefore, Daylight Savings Time is in effect for your school. Using Table 6-1 above, you see that you are four hours behind UTC (Eastern Daylight Time = -4 hours). Therefore you add four hours to your current time to get UTC time:

Convert first to 24 hour time format:	1:15 PM = 13:15 PM EDT
Now add the time difference:		13:15 + 4:00 = 17:15 UTC.
The date remains the same


Example 2: Your school is located in Minneapolis, MN, and it is March 1, 1994. The time is 10:15 PM.

Minneapolis is in the Central time zone. Since it is March 1, daylight savings time is not in effect. From Table 6-1, Central Standard Time is 6 hours behind UTC.

Converting to Military Time:		10:15 PM = 22:15 CST
Add 6 hours:				22:15 + 6:00 = 28:15

Since the result is larger than 24:00, it is after midnight on the next day, UTC (Its already tomorrow in England!). Therefore:

Subtract 24:00 from the time and add one to the day: 28:15 - 24:00 = 4:15 UTC on March 2, 1994.


Your school's pass is scheduled to occur at 22:37 on 5/17/1994, UTC. You are located in Denver, Colorado. What time will you tell the students and press that your pass will occur? (use Local Time)

Daylight Savings Time is in effect, and Denver is on Mountain Time. From Table 1, Mountain Daylight Time is 6 hours behind UTC. To obtain local time, subtract (you are going from UTC to local time) 6 hours from the UTC time to obtain local time:

Obtain Local Time: 22:37 - 6:00 = 16:37 or 4:37 in the afternoon.


The Shuttle will be visible to people in Los Angeles, CA, starting at 05:28 on November 1, 1994, UTC. What time will it be in Los Angeles? Los Angeles, CA is on Pacific Time, and since it is November, Daylight Savings Time is not in effect. Pacific Standard time is 8 hours behind UTC. 05:28 - 8:00 = (-3:28) Since the result is negative, the local date will be one day earlier than the UTC date. Add 24:00 to the UTC time and then subtract the difference: 05:28 + 24:00 = 29:28 --> 29:28 - 8:00 = 21:28 or 9:28 in the evening Subtract one from the day: 11/01/94 - 1 day: 10/31/94. The Shuttle will be visible to Los Angeles observers at 9:28 in the evening, on October 31, 1994.

Additionally, the following links provide excellent examples of converting between Local and UTC time:

MET To Local Time Conversions

Converting from MET to local time is a little more difficult. You will be given the launch time in UTC. You will be given your contact time and updates to your schedule in MET, so you will need to convert this to local time.


The Shuttle launches at 06:00:00 AM UTC on June 1, 1994. Your pass is scheduled for 04/13:00. You are located in Jackson, Mississippi. What date and time will it be locally?

Jackson, MS is in the Central Time Zone. Since it is June 1, Daylight Savings Time is in effect. From Table 6-1, the local time is 5 hours behind UTC. First, add the MET to the launch time and convert the MET to UTC. Then convert UTC to local time:

Start with the time: 	06:00:00 + 13:00:00 = 19:00:00
Now add the days: 	06/01/94 + 04/ = 06/05/94

The pass is scheduled to occur on June 5, 1994 at 19:00:00 UTC. Converting 
this to local time:

19:00:00 - 5:00:00 = 14:00:00 = 2:00 PM

The pass will occur on June 5, 1994 at 2:00 in the afternoon.
This is a simplified case. Most launches will not occur right on the hour, so a launch time of 13:31:00 is more likely. A short cut to this process is to convert the launch date from UTC to local time, and since it does not change, you do not need to go through the UTC to local time conversion each time.


The shuttle launches at 13:43 UTC on March 1, 1994. Your pass is scheduled for 06/09:55. Your school is located in New York City, NY. Compute the local time for your pass:

Convert UTC launch time to local time
March 1, New York City equates to Eastern Standard Time, for a time difference of -5 hours. Lift-off 
(MET 0/00:00) occurred at: 
13:43 - 5:00 = 08:43 EST on 3/1/94
MET 06/09:55 converts as follows:
08:43 + 09:55 =>
	Add minutes: 43 + 55 = 98 minutes = 1:38
	Add hours: 8 + 9 = 17 hours.
	Add minutes to hours: 
		17:00 + 1:38 = 18:38 or 6:38 PM
	Add days: 
		03/01/94 + 00/06/00 = 03/07/94.

The pass will occur at 6:38 PM on March 7, 1994.

Epoch Time Conversion Examples

Assume you have the following orbital element set, and you want to know how old it is. The best way to do that would be to convert it to local time and then compare to the current date.

1 16609U 86017A   97031.47844042  .00001345  00000-0  22290-4 0   103
2 16609  51.6513  53.2503 0011630  22.7100 337.4459 15.60476684625685

Satellite: Mir
Catalog number: 16609
Epoch time:      97031.47844042   
Element set:     010
Inclination:       51.6513 deg
RA of node:        53.2503 deg  
Eccentricity:     .0011630           
Arg of perigee:    22.7100 deg      
Mean anomaly:     337.4459 deg       
Mean motion:   15.60476684 rev/day   
Decay rate:      1.345e-05 rev/day^2 
Epoch rev:           62568       
Checksum:              268

Assume the local time 5:35 PM CDT on January 31, 1997.

First, from the Epoch Time of 97031.47844042, obtain the year and day of
the year:

	031 = January 31

Then multiply 24 by the fraction of the Epoch time (in this case: 0.47844042)
to obtain the UTC Hours: 

	24 x 0.47844042 = 11.48257008
	The UTC Hours is 11

Take the fraction portion of the UTC hours result and multiply it by 60 to
obtain the number of minutes

	60 x 0.48257008 = 28.95420480
	The UTC Minutes is 28

Take the fraction portion of the UTC Minutes result and multiply it by 60
to obtain the number of seconds and a decimal fraction of seconds:

	60 x 0.95420480 = 57.25228800
	The UTC Seconds is 57

Round the fraction portion of the UTC seconds to three decimal places to
achieve the thousand's of a second:

	0.25228800 rounded to 3 decimal places = 0.252

Assemble these components into the UTC time:

	 31-January-1997 at 11:28:57.252 UTC

Convert your local time to UTC:

	From Table 6, using Central Standard Time:
	11 Hours UTC - 6 Hours (UTC-CDT Difference) = 5 Hours CST
	UTC is 05:28:57.252

The local Date/Time of the Epoch is 5:28:57.252 AM on January 31, 1997, CST.
The current local time is 5:35 PM.  To obtain the exact difference, convert
both times to the number of seconds elapsed since midnight, subtract the
Epoch local time and then convert the difference back to military time:


	05:28:57.252 = (5 hours x 3600 seconds/hour) +
                       (28 minutes x 60 seconds/minute) +
		       (57.252 seconds)

		     = 18,000 + 1,680 + 57.252
	             = 19,737.252 Seconds

	5:35PM = 17:35:00 Military Format
      	17:35:00.000 = (17 x 3600 seconds/hour) +
		       (35 x 60 seconds/minute) +
                       (0.000 seconds)

		     = 61,200 + 2,100 + 0.000
		     = 63,300 Seconds

	Compute the difference in Seconds:

	63,300.000 - 19,737.252 = 43,562.748 seconds
	The Epoch Time was 43,562.748 seconds ago.  This doesn't really help
	you yet, so you must convert it to an MET-like format: DDD/HH:MM:SS.sss

	Obtain Days (DDD): 
		Divide 43,562.748 by 86,400.000 = 0.50419847
		The number to the left of the decimal point is the number of
		days that have elapsed.  In this case, DDD = 000

	Obtain Hours (HH):

		Take the fraction portion of the day calculation 
		(the remainder) and multiply it by 86,400 (the number of
		seconds in a day):

		86400.000 x 0.50419847 = 43,562.7478080
		Divide this result by 3600 (number of seconds in an hour)
		43,562.7478080 / 3600 = 12.10076328
		The Epoch was 12 hours ago.

	Obtain Minutes (MM):

		Take the fraction portion of the hour calculation above
		and multiply it times 3600 (number of seconds in an hour):

		3600 x 0.10076328 = 362.74780800
		Divide this result by 60 (number of seconds in an minute)
		362.7478080 / 60 = 6.0457968
		Minutes = 6

	Obtain Seconds (SS.sss):

		Take the fraction portion of the minutes calculation above
		and multiply it by 60 (number of seconds in a minute):

		60 x 0.0457968 = 2.74780800
		Seconds (rounded to the third decimal place): 2.748

The Epoch Time was 0 days, 12 hours, 6 minutes, and 2.748 seconds ago, or
expressed in an MET-like format: 000/12:06:02.748

Return to SAREX Field Operations Guide Main Page