Reference date is considered to be that date when 80 percent of the fruits sliced have hardened pits at the very tip plus 10 days.
Full bloom is considered to be that stage of bloom where a few petals have begun to fall.
Some years the bloom is sharp and easy to define whereas in borderline chilling years the bloom can be rather straggly. Heat is absorbed by the soil during the day and released to warm the blossoms at night and early in the morning. Maximum exposure of the soil to sunshine is necessary to provide optimum frost protection. Chilling quality varies. Warm, sunny days in January or December can cancel out the benefits of optimum low temperature chilling.