Time in Custom and Concept There are tensions between the customary way one thinks of time, and concepts of time in modern physics. Evolution has conditioned us (and probably many of our fellow creatures) to accept that time is a mysterious and unceasing progression through our consciousness of instants that separate the past from the future. We call the most important of these instants ‘now’ and accept for practical purposes that the world is what we see around us ‘now’ --- I’ll call it the natural ‘now’. This wysiwyg approximation has served sighted creatures like ourselves well throughout evolutionary history and is now a prejudice deeply ingrained in the perspective with which we humans have come to analyse our physical environment. We are also slow-moving (compared with light), tribal and chatter a lot. These attributes may have engendered in us our widely-held belief that natural now is the same for everyone, and that time flows evenly everywhere. But we have long been aware that this perspective is ambiguous when it comes to the quantitative assessment of time. Indeed it has probably been appreciated (since Rømer’s 1676 analysis of the motion of Jupiter’s moons) that because light travels with a finite speed your natural ‘now’ must in fact differ slightly from mine. The changing world we see around us does not look quite the same to both of us. And modern physics prefers the natural ‘now’. For a hundred years in physics ‘now’ has been defined as the instant where future and past light cones meet along an observer’s world line, as drawn on Minkowski diagrams in Einstein’s relativity. This physics ‘now’ is observer- dependent --- it is personal. For instance for antipodal persons at rest on the Earth’s surface physics ‘now’ differs by about 70 milliseconds. Nevertheless for practical purposes we accept that Greenwich Mean Time (GMT) can be used to export a standardized ‘now’ around the world, so that antipodal persons can standardize their ‘now’, in accord with the universally held belief that everyone’s natural now in the same. Here the tension between the custom of taking into account light-travel times (as in GMT) and our strongly held concept of a universal natural ‘now’ is resolved by compromise --- and the fact that it is not important enough to worry most folk. In physics, time has to do with more than one universally believed-in instant, more than one ‘now’. It has also to do with duration --- how time intervals are measured and compared in unfamiliar remote places and extreme situations. At a fundamental level physics --- or rather its theory of relativity --- uses the concept of the Local Inertial Frame, which to me seems to be a most ingenious and wise compromise between the concept of a personal natural ‘now’ (that of an observer at the coordinate origin) and standard GMT-like protocols for synchronising clocks and defining coordinates at points not at the origin --- elsewhere and elsewhen. With this compromise in place it turns out that both simultaneity and duration, as measured by relatively moving observers, each equipped with their own LIF’s, depend, firstly, on their relative motions (as in the well-validated theory of Special Relativity) and second: on the distribution of mass and energy (as in the elegant theory of General Relativity). It also turns out that the compromise concept of the LIF neatly resolves tensions (between the customary way one thinks of time and concepts of time in modern physics) but only on the mesoscopic scales of time and distance we move about in, and the tranquil backwater we inhabit. Here, and on these scales, the tensions are for most practical purposes weak enough to ignore. But when it comes to sorting out the information generated from observations of large scale happenings in violent and remote times and places, as in cosmology, the tensions become very evident. Fortunately the tensions are matters that concern the imagination, rather than our daily lives. We needn’t worry too much about whether time flows universally or whether we live in a block universe where past, present and future may coexist forever. Unless we are physicists. Do philosophers worry about such tensions?