Paraprogramming Dispatches

About Time

Eugene Zaikonnikov

This week I put together a small NTP client. To keep dependencies at minimum and to avoid forcing a permanently running process onto users, it does not attempt to adjust system RTC clock, compensate jitter or evaluate time server quality. As I see it, much of that behaviour is easy enough to add via mixins with the defined NTP class.

NTP timestamp is two 32-bit values: seconds and fraction of a second. NTP conveniently counts seconds from Jan 1 1900, just like universal time in Common Lisp. There is however no portable Common Lisp representation for fractions of a second. Thus the client sticks to using NTP formatted fraction for that. It is way more precision than any existing CL implementation has in INTERNAL-TIME-UNITS-PER-SECOND, but this makes the value comparable across implemenations. The new GET-ADJUSTED-UNIVERSAL-TIME method then returns a pair of values: universal time and NTP fraction. The fraction can be converted to the implementation’s internal time scale with FRACTION-TO-INTERNAL.

Internally we define no special arithmetic on NTP timestamps but provide two conversion macros for single integer space. BIG-TIME converts NTP stamp into a large integer. We then do all calculations in that domain, and convert back to NTP timestamp using SMALL-TIME when it’s time to send it over the wire. An NTP instance stores adjusted time as an offset from internal real time. The offset is roughly intialized with universal time and then adjusted after each server request.