The rule of time is a way of measurement and calculation time in order to unify time, it includes three elements: appointed unit of time, conventional origin of time and time calendar. The existing rules of time only apply to the earth and inside range of its gravitational potential, and not to the vast space out of the earth. For human beings to go to the space, it is an inevitable requirement to unify the time of space out of the earth. Here describes the definition and connotation of the time rule, and applies principle of general relativity to prove that the synchronicity should be only happened in the same coordinate system, and that does not have simultaneity between different coordinate systems. And then two expressions of time in the general relativity, proper time and coordinate time, were introduced. Coordinate time can be reproduced by pulsar looked like from infinite far away. Two modes to unify time were compared that one mode is “center time-keeping, local time service”, and another mode is “ local proper time, universal coordinate time”. The key point of this paper is to distinguish two kinds of viewpoint, absolute time view and relativity time view. And think that relative time viewpoint is the key to deal with the problem to unify time out of the earth. Three elements of time rules are explained by relativity time viewpoint again, and more, to put forward the concept of space time-keeping system.