What is the Accuracy of the Alpha Time of Year (BB_WATCH) Clock?
The specification for maximum clock drift in the Alpha hardware clock is 50 parts per million (ppm), that is less than ±0.000050 seconds of drift per second, less than ±0.000050 days of drift per day, or less than ±0.000050 years of drift per year, etc. (eg: An error of one second over a day-long interval is roughly 11ppm, or 1000000/(24*60*60).) Put another way, this is .005%, which is around 130 seconds per month or 26 minutes per year. The software-maintained system time can drift more than this, primarily due to other system activity. Typical causes of drift include extensive high-IPL code (soft memory errors, heavy activity at device IPLs, etc) that are causing the processing of the clock interrupts to be blocked. Also see Section 14.15, Section 4.3.
