The method of calculating [the beginning of] the seasons can be explained as follows. First, it is necessary to calculate the number of [nineteen-year] cycles that have passed until the [nineteen-year] cycle in question. Afterwards, add one hour and 485 units for every [nineteen-year] cycle. Afterwards, group all the units into hours, and all the hours into days. [Once a] total [has been reached], subtract seven days, nine hours, and 642 units [from it]. Add the remainder to [the time of] the conjunction of Nisan in the first year of the [nineteen-year] cycle in question, and you will be able to know the hour and the date of the the vernal equinox of the first year of this cycle. From this date, you can calculate [the beginnings of] all the subsequent seasons [by] adding ninety-one days and seven and one- half hours for every season.
If you desire to know [the time and the date of] the vernal equinox of a particular year within a given [nineteen-year] cycle, [the following procedure should be used:] Add one hour and 485 [units] for every [nineteen-year] cycle. For each complete year that has passed within the [nineteen-year] cycle [under discussion], add ten days, twenty-one hours, and 204 units,, and then group the entire sum [into days and hours].
Afterwards, subtract seven days, nine hours, and 642 units [from this sum] and divide the remainder into lunar months of 29 days, 12 hours, and 793 units. [The number of days, hours, and units that] remain [after all the complete] lunar months [have been calculated] should be added to [the day and the time of] the conjunction of Nisan in that year. [In this manner,] you will be able to determine the date and the time of the vernal equinox of the year desired.
According to this calculation, the vernal equinox will always take place either at nightfall, at midnight, at daybreak, or at noon. The summer solstice will always take place at either 7:30 PM, 1:30 AM, 7:30 AM, or 1:30 PM. The autumnal equinox will always take place either at nine or at three o'clock, either in the day or the night. The winter solstice will always take place either at 10:30 PM, 4:30 AM, 10:30 AM, or 4:30 PM.
If you desire to know the day of the week and the hour of the equinox, [the following procedure should be used:] Count the number of complete years that have passed from the year of creation until the desired year, and divide them into groups of twenty- eight. Add one day and six hours for each year remaining. Total the sum [of the hours and the days], and then add three days. Afterwards, divide the days into groups of seven. The remainder of the days and the hours should be added to the time of nightfall on the first day of the week. The result will be [the day and the time] on which the vernal equinox will occur.
Why is it necessary to add three days? Because the first equinox of the year of creation took place at the beginning of the fourth day.