'Edge of time' meaning of phrase

They were lost in their own world and time itself was irrelevant

Edge:

• a line or border at which a surface terminates.

Here edge of time is used in a figurative way to refer to the far end of time suggesting that they were so immersed in their chatting in office that the house appeared to be as something very far away from their reality. The author is probably suggesting that they were so absorbed by their chat that they almost lost touch with they world.

It's metaphorical, suggesting a place where time no longer flows.