466 II. 6
Being and Time

And because the temporality of that Dasein which must take its time is finite, its days are already numbered. Concernful awaiting takes precaution to define the 'thens' with which it is to concern itself—that is, to divide up the day. And the 'during-the-daytime' makes this possible. This dividing-up, in turn, is done with regard to that by which time is dated—the journeying sun. Sunset and midday, like the sunrise itself, are distinctive 'places' which this heavenly body occupies. Its regularly recurring passage is something which Dasein, as thrown into the world and giving itself time temporalizingly, takes into its reckoning. Dasein historizes from day to day by reason of its way of interpreting time by dating it—a way which is adumbrated in its thrownness into the "there".

This dating of things in terms of the heavenly body which sheds forth light and warmth, and in terms of its distinctive 'places' in the sky, is a way of assigning time which can be done in our Being with one another 'under the same sky', and which can be done for 'Everyman' at any time in the same way, so that within certain limits everyone is proximally agreed upon it. That by which things are thus dated is available environmentally and yet not restricted to the world of equipment with which one currently concerns oneself. It is rather the case that in the world the environing Nature and the public environment are always discovered along with it.ii This public dating, in which everyone assigns himself his time, is one which everyone can 'reckon' on simultaneously; it uses a publicly available measure. This dating reckons with time in the sense of a measuring of time; and such measuring requires something by which time is to be measured—namely, a clock. This implies that along with the temporality of Dasein as thrown, abandoned to the 'world', and giving itself time, something like a 'clock' is also discovered-that is, something ready-to-hand which in its regular recurrence has become accessible in one's making present awaitingly. The Being which has been thrown and is alongside the ready-to-hand is grounded in temporality. Temporality is the reason for the clock. As the condition for the possibility that a clock is factically necessary, temporality is likewise the condition for its discoverability. For while the course of the sun is encountered along with the discoveredness of entities within-the-world, it is only by making it present in awaitingly retaining, and by doing so in a way which interprets itself, that dating in terms of what is ready-to-hand environmentally in a public way is made possible and is also required.

Dasein has its basis in temporality, and the 'natural' clock which has already been discovered along with Dasein's factical thrownness furnishes [414] the first motivation for the production and use of clocks which will be somewhat more handy; it also makes this possible. Indeed it does this in such a manner that these 'artificial' clocks must be 'adjusted' to that

Being and Time (M&R) by Martin Heidegger