At a first glance somebody could say "…..how is it possible ? A regulator fitted with a calendar.
Can we expect it will be precise? ….. we can! Let's go step by step.

A regulator is, by definition, a clock we consider precise for its movement is not encumbered by other "complications" , be that a simple chime, ending with anything more complex.

We do not want to disturb in any way the operating smoothness of the clock.

As a matter of facts every energy shift needed to give motion these complications is at least a change of energy given by the escapement to the pendulum, and this might mean a change in its swing amplitude ,... with devastating results in the clock ability to keep precise time.

	Gravity escapements, those of the type invented by Lord Grimthorpe , the designer of Big Ben , do not substantially suffer of this drawback ,for the escapement accomplishes its function in just lifting two gravity arms. Once lifted and then freed from their stops, these arms ,leading the pendulum, confer it the impulse which can be regarded as constant for it is due to their weights only . Invented a constant force escapement so reliable as to let us to build a precise clock?
first stage of the scape wheel		cutting the arms of the scape wheel

Unfortunately, no!

Even if the gravity arms while giving the impulse are in contact with the pendulum ( this being regarded by someone with suspicion ) ,we could say its influence is constant, hence negligible.

But, it should not be forgotten it is the pendulum itself which during part of its oscillation unlocks the gravity arms allowing them to fall down. Also, friction which could occur in this unlocking action may vary with change of, say, lubrication of the sliding planes or environmental conditions.

And we, clock enthusiast, regard with suspect the word "variation ".

Arnfield overcome this problem with a variant of gravity escapement ,which, I believe is one of the most reliable.

Back view		escapement wheel
	side view