UTC, Local, and Epoch Time

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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.
**

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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.

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.

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.

UTC/Local Time Zone Differences

TIME ZONE |
STANDARD TIME |
DAYLIGHTSAVINGS TIME |

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:

- DDD is the ordinal number of the day since January 1 of the current year (UTC Midnight, January 1).
- HH is the UTC Hour
- MM is the UTC Minute
- SS is the UTC Second.

- YY is the two digit year
- DDD is the ordinal number of the day since January 1 of the current year (UTC Midnight, January 1)
- fraction is the time of day, expressed as an eight signficant digit decimal fraction. Midnight at the start of the day is .00000000 and noon is .500000.

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

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.

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.

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

- (Local to UTC)Conversion Examples
- (UTC to Local)Conversion Examples

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.

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.

Mir 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: 1997 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: EPOCH 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 CURRENT LOCAL TIME: 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