Ekam = One-um
Samaya = Time
Sam = One, once, with, con, co-;
Ayam = PED: "...refers to what is immediately in front of the speaker (the subject in question) or before his eyes or in his present time and situation, thus often to be trsld. by "before our eyes", "the present", "this here", "just this"
A = to, a;
Ya-m = whatsoever-um.
The objection by some to the translation "Once Upon A Time" is that it is used in Children's stories. The response to this objection: (Ignoring for the present the idea that on the one hand, in most cases, including the one under discussion, these suttas are being addressed to what is no more than a child-like mind, and on the other hand, that a child-like openness of mind is exactly the state that is best receptive to these teachings...) The fact is that the use in these old suttas came first, the use in children's stories later. So the fault, in hearing this as an opening to a children's story lies with the listener, not the translation. The listener is hearing a "stock phrase"; the translation is of a statement of fact. "At one point in time." At a location in time (usually followed by the location in space).