I’m posting this from the Quora.com site to save the explanation of what relationships are all about, what systems in the universe are all about, and the, perhaps, most important aspects of such systems.
Nothing, including space-time, exists except if there is a relationship.
What is the simplest of relationships in our universe? The simplest of relationships is represented by a near locality (near position), a near singularity, a near point. Why do I use the word “near?” Because if there were a pure singularity source in the universe, a pure locality, a point, then there would only be one thing, one magnitude, and for something to exist (in our physical universe), it must be in relationship with something else: there must be a change, a change boundary (for example: temperature changes across it, when temperature changes, we call that relationship/relationship-boundary/change-boundary “heat.”)
The size of a relationship boundary at any time represents our universe (what can be, or exist, or behave, or experience it).
Our universe is filled with potential, but it doesn’t exist explicitly (only implicitly: see works of David Bohm) when relationship boundaries are condensed out of it.
Many ask where is the outer boundary, when they think of the expansion of the universe as a giant balloon being blown up. But universal boundaries (universal relationship boundaries) exist (us) inside a mathematical set of statistical potentials. Statistical potentials determine the probabilities of the physical laws of our universe.
So, boundaries become more and more complex. The particles condensed therein relate to form atoms (atomic-tensions/atomic-fields), molecules (molecular-tensions/molecular-fields), all the way to the experiences of cellular cephalization.
All systems (relationship change boundaries) in our universe have at least four major INDIVISIBLE characteristics: FORM (geometric shape/size/curvature), FUNCTION (behavior/spacetial-path), DURATION (length of path in time), and EXPERIENCE.
Humans use clocks (external to them) to objectively measure their experiences. But most systems have a way of internally measuring theirs. Our cells go through day and night cycles that help them function effectively.
To answer your question, based on the above understanding: TIME/DURATION started at the same time form and function started: at the very beginning of the universe, because these major aspects of the universe (universal subsystems): FORM/FUNCTION/DURATION/EXPERIENCE spontaneously existed all together—all at once—somehow fused!