Time, personified, like Chronos. I don't know which one reflects Tolkien's own perception of Time, how could we know? But it certainly shows the differences between the earlier and later versions of the world he created. The latter I think is more simple, leaving space for the reader to think about and discuss the nature of Time in his work, so is preferable to me.
I'm not sure that having Eru outside Time but living alongside him works?