Northumbria in Bede's childhood had two churches keeping Easter on different Sundays. The Irish mission that had converted the north used an older table for calculating the date; the Roman mission used the Alexandrian nineteen year cycle. A king could be feasting while his queen was still fasting through Lent. The Synod of Whitby in 664 decided for the Roman reckoning, and the technical discipline behind that decision, computus, became Bede's life work.
Computus is astronomy done for a liturgical purpose. Easter is fixed to the first Sunday after the first full moon on or after the spring equinox, which means predicting lunar months against solar years indefinitely into the future. Bede wrote a short handbook, De temporibus, in 703, and then the full treatment, De temporum ratione, in 725. It covers finger reckoning, the divisions of time, the moon's course, the leap year, the ages of the world, and one section that has nothing to do with Easter at all: a correct account of the tides as following the moon, including the observation that the time of high water differs along a coast.
The same discipline produced the dating system. Dionysius Exiguus had proposed counting years from the incarnation two centuries earlier and it had gone almost unused. Bede adopted it in his Easter tables and then throughout the Ecclesiastical History, and also counted backwards before it, which is the first sustained use of a scheme now in worldwide civil use.