GMT Seconds

From ASPMHelp
Jump to: navigation, search

Why Seconds?

For computational convenience, ASPM stores time as GMT Seconds since 12:00 am January 1, 1980. Most Unix and Unix based systems use 1970, but at the time we saved several digits by starting 10 years later. Also, in the early 1990's disk space was still quite expensive and a few bytes times a few millions could add up quickly.

Also, since dates and times are handled differently by different operating systems and ASPM wanted to have a portable format, we chose to store time as an integer in seconds.

Why GMT Time?

Most aeronautical data is kept in some type of universal time to allow calculations free of local time issues. We chose GMT time because at that time, it was a fairly accepted time and most folks understood calculating local time using an offset from GMT time. This makes calculations very easy since to find out how long a flight was in the air, you could simply subtract wheels off from wheel on to get airborne. ASPM handles all time calculations in this GMT Seconds and display in local time as needed.

Calculation Tips

GMT Seconds make use of the fact there are 86,400 seconds in a day. So, to find the day from GMT Seconds, you would add GMT Seconds / 86400 to 1/1/1980. To find the time, you would take the remainder and divide by 60 to get minutes into the day or 3600 to get hours into the day.

If you are using a spreadsheet or many programming languages that manage dates as ffloating point numbers, you may be able to simply add the seconds / 86400 to 1/1/80. For example, in Excel you can enter the formula as "1/1/1980" +F4/86400 where F4 is a cell with GMT Seconds.