The equation of time is the difference between apparent time, what you see on a watch, and solar time, the actual length of a day based on the earth's orbit around the sun. Throughout the year, the equation of time (apparent time minus solar time) ranges from about -14 minutes on 11 February to +16 minutes on 3 November.  Only on 15 April, 14 June, 1 September and 24 December is the day exactly 24 hours long.

There are two reasons for that. One is because of the earth's elliptical orbit around the sun, meaning it traces an ovoid shape around the sun, instead of the perfect circle we normally see. Less well known is the fact that the earth's own axis of rotation is at an angle relative to its orbit round the sun.

Needless to say, the clever folks in Switzerland quickly put the equation of time onto a watch. This complication was first seen on pocketwatches as old as 150 years, but now it is widely seen on wristwatches, almost always with perpetual calendar function. There are two ways of indicating the equation of time, either a subdial indicating the difference in solar and apparent time, or with an additional minute hand that displays solar time. Daniel Roth has taken the former route in creating their Perpetual Calendar with Equation of Time.

First unveiled in 2004, the Perpetual Calendar with Equation of Time was given a face-lift this year, although the movement remains the same. The perpetual calendar module is manufactured by Daniel Roth, reflecting its strength in complications development; the base movement is a Girard-Perregaux cal. 3100. Girard-Perregaux makes a similar model in the Vintage 1945 case, but tellingly the movement is created by Daniel Roth.

Aside from the equation of time and perpetual calendar, this watch another clever complication - a days of the month indicator. A hand points to the number of the days in the month, 28, 29, 30 or 31; simple enough. But that belies the complexity of the mechanism behind the display, which has to account for leap years in addition to individual months of common years. This is far more practical than the leap year calendar usually integrated into perpetual calendars; with this you can see at a glance whether February has 28 or 29 days.

I give the 2007 Perpetual Calendar with Equation of Time high marks for the winning design; it is definitely one of the most attractive models in the current collection. The designers at Daniel Roth succeeded in combining a disparate array of design elements - spiral guilloché, Roman numerals, a realistic moonphase and a partially skeletonised dial - into a handsome watch. Of the 5 different perpetual calendar watches Daniel Roth makes, this is my favourite (it used to be the instantaneous perpetual but after seeing this that falls to a close second).

What do you think of the design?

Perpetual Calendar with Equation of Time in white gold

- SJX