Date

An event such as the arrival of a shipment or the inspection of a product occurs at a point in time. Accurate time and date measurement are critical for industry and commerce. Shipping, delivery, receiving, transport and expiration dates must be carefully represented, recorded and transmitted to many interested parties including manufactures, distributors, retailers and consumers.

Clearly there are many ways to represent time and date - years, months, days, hours, minutes and seconds reference from 24 separate time zeros. There are also various methods for storing and displaying this information: June 1, 2001; 7-1-2001; 2001.7.1; Fri Jun 1, 2001 19:39:30, etc.

Consistent with our philosophy of a simple representation for automated transmission, we propose a single time standard. Time is a duration measured from a given reference point. The selection of this reference point is somewhat arbitrary, but its existence is vital. Therefore we propose all times are measured from January 1, 1970 midnight Greenwich Mean Time. (This is the standard used by the UNIX operating system, and therefore a standard familiar to a generation of programmers, operating systems and applications.)

Units for time vary. Age is measured in years, plane flights in hours, television shows in minutes and stoplights in seconds. However, we propose a date measure in milliseconds. This should provide more than enough accuracy for most time measurements relevant to commerce and industry, yet not so much that transmission and storage of dates are prohibitive. Thus the time measurement in PML is the number of milliseconds that have passed since January 1, 1970 00:00:00.000 GMT.

Times may assume both positive and negative values allowing measurement before January 1, 1970 - though presumably most PML files will produce dates after this date. Representing times with a signed 64-bit integer will provide date measurement until the year 292,270,993. That should be sufficient.