How did they write dates back in the middle ages?

Answer 1

by using feather pens and a bowl with ink

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Answer: I believe you are asking how dates were calculated rather than the mechanics of how they were written down.

The answer is that various different systems were used.

• Royal charters and legal documents always refer to the year of the king's reign: "In the 5th year of King Stephen" for example. The famous Magna Carta ends not with King John's signature (as many people wrongly believe) but with the dateline: "Given by our hand in the meadow that is called Runnymede, between Windsor and Staines, on the fifteenth day of June in the seventeenth year of our reign", meaning 15 June 1215.
• The Church calendar was far more complex and used the old Roman system of Kalends, Ides and Nones together with the feast days of the saints to specify any particular date. It also used the term "octave" to refer back to a feast day a week earlier ("octave" means an eight day period but actually means the 7 days after the feast day, since the day itself is also included). So 2 days after the feast of St Michael would be 10 May, the Octave of St Ranulf would be 3 April and the Kalends of June is 1 June. Medieval years were calculated to start from either 25 December or 25 March, not from 1 January.
• Ordinary folk would have little concept of dates, apart from the major Holy Days, Sundays and feast days that were announced by the local priest. They would express dates only roughly, for example as "a few days before Martinmas".

Answer 2

Ex:

OF THE TEWNTY-THIRD OF MAY AND THE YEAR OF THIRTEEN HUNDRED AND FIFY-SIX