Albert Einstein is quoted as having said "Zeit ist das, was man an der Uhr abliest" ["Time is what a clock measures"]. The question is, as per the title of the thread, how exactly does a clock measure time? When considering this question we need to consider a few things; namely: - what time is considered to be in the physical sciences - how the physical processes of a clock measure a secondary physical property called time (if that is what time is asserted to be) - how a clock demonstrates that there is a temporal dimension i.e. how it demonstrates the relationship between past and present, or present and future. Time in physics The first point is something I'm not abundantly clear on. Does time exist in Quantum Gravity? In Einsteinian relativity, time appears to be physical, dynamical, as well as relative; motion affects its passage for different frames of reference. It appears to be different in Quantum Mechanics, where it appears to take on a more absolute form. That appears to be similar to the view under the interpretation of Lorentzian relativity, which is equally supported by relativistic experiments, as Einsteinian relativity. All three view time as a physical property of the universe - or so I believe. Measurement A clock provides a regularly occurring, repetitive process which is used for the purpose of comparison. The repetitive process provides a standard unit in which other, different processes are expressed, and then compared to yet other processes, expressed using the same standard units. For example, if we take the standard atomic clock, the recurring process there is the oscillations of the caesium-133 atom; what appears to be actually measured, by the atomic clock, is the number of oscillations of the atom, not some secondary physical property called time. Theses oscillations are then used to compare different processes. For example; if we say that an object is displaced by a distance of X in 1 second, what we actually mean is that when 9,192,631,770 oscillations are counted, the distance traveled by the object will be X. We can then compare other objects using this standard unit of comparison; if an object is displaced by a distance of X+2 when the counter of the clock reaches 9,192,631,770, then we say that the second object has moved more quickly than the first. The question is, at what point in this process is a secondary (or tertiary), physical property called time measured - without, of course, simply assuming that it is? "Distance" "Distance is what a ruler measures" is a comparison often used to attempt to explain how a clock measures time, but, much like time, "distance" is just a concept. If we talk about measuring the "distance" between ourselves and a remote object what we are actually saying is how many standard units of measurement - a metre stick for example - could we fit between [an arbitrarily defined point on] ourselves and [an arbitrarily defined point on] the object. If the object we are talking about is a coffee table in our living room for example, we might say how many metre sticks can we lay between ourselves and the coffee table; that number would correspond to the amount of floor between us and the coffee table. While we might say the floor exists, the coffee table exists, we exist, and the metre stick exists, "distance" is just a concept. Dimensions "Length is what a ruler measures" is somewhat different to the notion of a ruler measuring distance, because here we are talking about the physical dimensions of an object, as opposed to the conceptual distance between objects. Time, however, is somewhat different to the spatial dimensions of an object; the three spatial dimensions are [in general] clearly observable, however, the question is how do we discern that an object, or a process, has a temporal dimension? The time co-ordinate of an object, or a process, will always be "now"; that is, any attempt to measure a temporal dimension can only be carried out in the present moment. While we may be able to recall a previous state of an object or process, this recollection is just a mental construct, a memory; we may also be able to project a future state of the object, but this too is just a mental contsrtuct, or a concept. The same applies to any mathematical representation of the "past" and "future" states. Given that this same reasoning applies to any clock, how can a clock demonstrate that there is a temporal dimension? Conclusion A conclusion that could be drawn is that time is not actually a physical property; it is, however, dynamical and relative but only insofar as it is a mental construct for each individual, and our memories and projections can be distorted. It appears as though "time" is not so much something to be measured, as it is the system of measurement, or comparison. All that, of course, is based on the reasoning that a clock does not actually measure a physical property called time; but the question remains, how exactly does a clock measure time?