Let's begin with this premise, that charters and manorial records, when they come to have an internal date, have one which assumes the form 'On the Wednesday next after the feast of the Nativity of St John the Baptist in the 3rd year of King Edward son of King Edward'. The date thus consists of several elements:

  • day of the week;
  • relationship to a feast day;
  • a regnal year (but some
    manorial records, particularly
    of large Benedictine religious
    houses, might have an abbatial year,
    or some other acta might
    have an episcopal or papal year).

Days of the week
Monday dies lune
Tuesday dies martis
Wednesday dies mercurii
Thursday dies jovis
Friday dies veneris
Saturday dies sabbati
Sunday dies dominica

Let's then take these elements separately as they occur and then reconstruct the whole at the end.

Reference for Saints' days, Easter Tables and regnal years:

C. R. Cheney, Handbook of Dates for Students of English History (Royal Historical Society, any edition). The following discussion examines how to use the sections of Cheney sequentially to find the precise date.

  • First consult the table of saints' days;
  • then check the regnal years for the year of grace in which the feast fell;
  • then check the Easter Tables for the date of Easter in that calendar year ;
  • then check the calendars for the appropriate Easter date to find out on which day the feast fell and thus the date of the day on which your document was compiled.

    This process is described in more detail below.

Let's convert on the Tuesday before the Feast of St Michael the Archangel 9 Edward III (die martis proxima ante festum sancti Michaelis archangeli anno regni regis Edwardi tercii nono)

1 In the table of saints' days, Michaelmas is immoveable at 29 Sept.
Cheney, Handbook, p. 56
2 The regnal year 9 Edward III ran from 25 Jan. 1335-24 Jan. 1336.
Cheney, Handbook, p. 20.
Therefore this Michaelmas fell in 1335.
3 The Easter Tables specify that in 1335 Easter fell on 16 April.
Cheney, Handbook, p. 158
4 The calendar for the year in which Easter fell on 16 April reveals that Michaelmas fell on Friday in that year, so the Tuesday before was the 26th Sept. [1335].
Cheney, Handbook, pp.134-5.
Try to follow this through in Cheney, Handbook of Dates for Students of English History

Self-assess on chronology

In general, but not exclusively, royal charters are not dated by a regnal year before the accession of Richard I (i.e. not before 1189). Before that date, their 'date' consists of a place-date -- Given at such and such a place (Data apud...).
In general, but not exclusively, private charters do not have any form of date before the very end of the thirteenth century -- that is a general proposition because there is a significant minority of privatae conventiones which do have a date, even a year of grace, even in the late twelfth century.
For the privata conventio or private charter, even if it is still undated, it can sometimes be established whether it is before or after 1290 by whether it contains the Quia Emptores clause.
In general, but not exclusively, as was shown by DeLisle, the royal style Dei gratia was more consistently introduced by Henry II after May 1172-May 1173. To be continued