In your original example you did not specify what CAT A time frame is used.
As an example for an FDR fault the MEL will state a limitation on the time say 3 flight days This means if the aircraft remains on the ground for a week, you could then operates the aircraft every other day in this case the MEL entry would be valid for 13 days You can use any combination you want but the bottom line is that limitation is based on flight days
However if the MEL stipulates 6 calender days then at the end of this period it must be fixed or a concession obtained.
It is only applicable to CAT A item because all other categories are based on calender days only
Regarding service checks the rules are black and white on this if a check will expire in flight the check must be done before it takes to the air unless you obtain an extension to the check such that it expire on the ground