For fundamental questions on time you must refer to proper time, not to coordinate time. That means: Observers are measuring with their respective clocks time periods of worldlines. From such results in the form of coordinate time may always be retrieved the corresponding proper time of the corresponding worldline.
We can distinguish two kinds of worldlines: The spacetime interval of the worldlines of lightlike phenomena such as massless particles and fields is always zero, so they will not help us to understand what (proper) time is. In contrast, mass particles have worldlines with proper time. So when we want to understand what time is, it is sufficient to consider the proper time of mass particles.
The proper time of mass particles is a sort of "pulsebeat" of the particle. The particle has a certain frequency of aging. And this is what (proper) time is. In a second step, we may observe this pulsebeat of the particle, and the respective pulsebeat of all particles. However, the coordinate time we use is the proper time of each particle multiplied by the factors of time dilation - proper time is the time before time dilation, and coordinate time is the time after time dilation.
What we get is our observation, that is a complete coordinate manifold of time measurements of the universe. But each coordinate time measurement can be reduced to the proper time of the corresponding particle.
In one word: There is no common time axis, time is found separately within the different particles, it is their respective duration, their respective aging frequency and their respective proper time.