The substance of time is motion and change:
a static world would also be a timeless world. Nevertheless,
time, as forged by its Creator, follows a seven-day cycle,
consisting of six days of work (i.e. development, transformation,
change), and a seventh day of rest. But how can a unit of
time be defined as a day of rest, the very antithesis
Time and space are closely related to each other; indeed,
modern physics is wont to combine the two as spacetime,
a four-dimensional grid against which all physical objects
and events are measured. While time is the more abstract of
the two, many of the characteristics of space are attributed
to time as well: we speak of a point in time,
a stretch of time, time cycles, even
of the condensation or fanning out
Indeed, many of time's complexities can be better understood
when we apply the space-quantifying models of geometry to
our conception of time.
One such model is the circle, long considered the most perfect
of spatial shapes. The circle's primary features are: the
center, the point from which the circle's area extends
uniformly in all directions; the radius, which is the
distance from the circle's center to its outer limit; and
the circumference, the circle's outer rim, which contains
the area of the circle within it. The circumference is (approximately) six times the length of the radius. This is
true of all circles, regardless of size: the greater the radius,
the greater the area of the circle, and thus the greater the
circumference that encloses this area; but the proportion
always remains the same---each additional inch (or yard or
mile) of radius will translate into (slightly more than) six
additional inches (or yards or miles) of circumference.
The geometric point possesses no area; as such, it would
seem, it hardly qualifies as a component of space. In truth,
of course, the very opposite is true: the point is the most
basic component of all geometric forms---every line is defined
by the points that mark its beginning, end, center, convergence
with other lines, etc.; and every area is defined by the lines
that frame it. Indeed, it is precisely because the
point possesses no area itself that it can define and quantify
the areas that relate to it.
This is exemplified by the circle's center. A mere
point, the center occupies none of the circle's area; but
it is precisely the center that makes the circle a circle.
The radius extends from it, the diameter turns on it, the
circumference is drawn in relation to it---virtually every
feature and characteristic of the circle is derived from the
point upon which it is centered.
To understand the week--the 7-day cycle which the Creator
stamped into the very fabric of time--we must envision it
as a circle. The exterior surface of this circle (its circumference)
is the six work days---days that are tracts of time, expanses
of progression and change. The center of the circle is Shabbat.
Shabbat is a timeless point in time, an island of tranquility
in a sea of flux. Yet despite--indeed, because of--its timelessness,
Shabbat is the axis upon which the most basic of time-cycles,
the week, turns.
For Shabbat is the day that embodies the purpose and end-goal
of time---the objective of all the work, development and change
in our restless existence. On Shabbat, our efforts of the
past six workdays translate into a holier and more G-dly world,
a world brought that much closer to the harmonious perfection
that G-d imbued in creation and charged us to develop. One
day a week, we penetrate below the whirling surface of time
to experience its tranquil core. One day a week, we enjoy
a taste of the world to come, a taste of an age
that is wholly Shabbat and rest, for everlasting life---the
eternal Shabbat of Moshiach that we are striving towards.
And it is this weekly taste of tranquil perfection that supplies
us with the vision and fortitude to grapple with and transform
the still imperfect world we return to during the six workdays
of the coming week.
Shabbat, then, is both the source and the goal of the six
days of conventional time that form the surface
of the weekly cycle. It is thus the very essence of time,
precisely because it is devoid of the motion and flux which
characterize time---in the same way that the area of the circle
derives from and is defined by its center-point, precisely
because of the center's own arealessness.
In the circle of time, the distance from center to surface
is multiplied six-fold, resulting in a "circumference''
that is six times the radius: six workdays result
of the journey from the tranquil center of the week to its
whirling. For life is a six-dimensional affair, reflecting
the six divine attributes (sefirot) that G-d invested
in His creation. Thus we have six days of creation, with Shabbat
as its timeless core; we have the six directions
of space, and the area less center from which they extend;
we have the six basic attributes of the heart (attraction,
rejection, synthesis, competitiveness, devotion and comunicativity), and the seventh attribute, malchut (receptiveness),
which, while possessing no qualities of its own
is the focal point and implementer of them all. The entirety
of existence is comprised of these six basic elements, coloring
every endeavor and experience with the six basic hues of reality.
Another law of circle geometry is that the greater the radius,
the greater the circumference. The same is true of the circle
of time. The further we depart from its timeless center, the
more body time has: the more turbulent it is,
the more at odds with the Shabbat at its core.
But no matter how great the surface flux of our life, it
is inexorably bound to the tranquil axis, deriving from it
and tending to it. Ultimately, the most tumultuous periods
of our lives are generated by its quintessential purpose and
serve its harmonious end.
Based on an entry in the Rebbe's journal
Adapted from the teachings of the Rebbe by Yanki
 See Derech Mitzvotecha, 59a: In the chambers
of Yetzirah (a spiritual world which precedes our
physical universe in the chain of evolution
from spirit to matter which G-d employed in His creation),
fifteen years (of our time) are included in
a single glance.
 The exact figure has yet to be calculated; mathematicians
today believe that the ratio of a circle's circumference
to its diameter (a circle's diameter is its width from end
to end, or a line exactly double its radius) is an endless
string of numbers that begins 3.14..... (represented by
the symbol Š (pi)). According to this, the circumference
of a circle is within a hundredth part of 6.28 times its
Halachically, the Talmud (Eruvin 13b) rules that
[a circle] whose circumference is three tefachim,
has a width of one tefach.'' Maimonides, in his commentary
on the Mishnah (ibid.), explains: Know, that the ratio
of a circle's diameter to its circumference is not known
and can never be expressed absolutely. This lack of knowledge
is not due to our [ignorance]... for this thing is, in essence,
unknowable. It can, however, be known approximately... the
closest figure [that has been arrived at] is approximately
three and one seventh. Since the exact figure cannot be
known, [the Talmud] adapts a round figure, ruling that,
regarding all measurements required by the Torah, that
which has a circumference of three tefachim, has
a diameter of one tefach.
 As one of many possible translations of the kabbalistic
terms chessed, gevurah, tiferet, netzach, hod and
yesod would have it.
 Reshimot #3 pp. 46-47.