The first three words are:
computac(i)o(n)e eccl(es)iae Anglicanae
There is a fourth word beginning with c. I'm not sure of this one.
The first three words essentially mean: by the computation of the English Church
This refers to the date. I think the key issue referred to is the year beginning on Lady Day (March 25).
The message above is shortened even further, due to limitation of available space.
ADDED:
The dates here which are shown to have the computation message are 11 February and 13 March, both of which are in the period of the year where the year part of the date will vary between Old Style dating (year begins on Lady Day) and New Style dating (year begins 1 January).
In this period the Old Style year lags the New Style year by 1.