xerox Alto Time Standard Feb78 free download

File name Alto_Time_Standard_Feb78.pdf

Inter- Office Memorandum To PARC/SDD Date February 26, 1978 From Ed Taft Location Palo Alto Subject Alto Time Standard Organization PARC/CSL (Edition 3) XEROX Filed on: AltoTime.Ears, .Press, AltoTime.Bravo I am resurrecting a proposal I made nearly two years ago to change the Alto's internal date and time standard. The need for this change has become more pressing since that time, so I would like to see it accomplished as quickly as possible. A number of software systems will be affected by the change. This memo attempts to enumerate them, but I have undoubtedly missed some (particularly ones in the Mesa and Smalltalk worlds). Fortunately, the consequence of failing to convert immediately is not catastrophic. The last section of this memo sets forth a schedule for converting from the old standard to the new. My intention is that the conversion be relatively smooth and that old programs continue to work until such time as support for the old standard is revoked. Change since Edition 1: The internal representation of a time zone is entirely different so as to accomodate fractions of an hour and other oddities. Change since Edition 2: Current status. Present Standard The current Alto time standard is a 32- bit integer denoting the number of seconds since midnight, January 1, 1901, local time (with Daylight Savings Time already applied if it's that time of the year). This standard has two important difficulties. First, it is location- dependent: a given 32- bit integer represents a different absolute time depending on the time zone within which it is interpreted. With the proliferation of Altos across many time zones and the certainty that they will eventually be able to communicate with each other, it becomes desirable that a time standard be location- independent. Given this, a host in one time zone may obtain the current date and time (as a 32- bit number) from a host in another time zone, without either host having to know the location of the other. This capability is needed now because Webster has just been connected to our present Parc/SDD/XEOS inter- network. Second, the present time standard is not monotonic: it jumps forward an hour in April and backward an hour in October so as to conform to daylight and standard time. Hence an Alto's clock cannot be maintained correctly simply by counting milliseconds, and for this reason it is not maintained correctly at present. Furthermore, there is a two- hour stretch of absolute time every October within which each 32- bit num