The Saudi calendar as regulated by the Majlusl Qadhaa a’Alaa was based on a 32 year pre-set calendar set in accordance with the Greenwich New Moon Astronomical Timetable from 1409 AH to 1440 AH. This resulted in the month commencing approximately 1.5 days prior to the visibility of the moon or some 8 – 9 hours before its own new moon time.
To explain, the Saudi calendar was set according to GM Time which starts at 12 o’clock midnight, corresponding to 3am Saudi time. So, if the new moon is shown to be just before 12 o’clock midnight in Britain, then the next morning is counted as the first of the next lunar month in Saudi. Once they announce that the first is from the morning, it implies, that the moon was sighted at Maghrib the previous night. If we presume Maghrib to be at 6 pm, then the announcement, in Saudi Arabia has come some 9 hours before their new moon time. Because the new month should commence with visibility time, the Saudi calendar begins approximately 1.5 days early.
Subsequently, since 1420AH, the Saudi calendar was adjusted to the new moon time. This calendar still falls short of the actual visibility time, which is some 18 hours after the new moon time resulting in the calendar still being one day ahead.