Full Idea
McTaggart says the A-series is more fundamental than the B-series. An objective being could not deduce the present moment of the A-series from the B-series, but the B-series can be deduced from the A-series.
Gist of Idea
The B-series can be inferred from the A-series, but not the other way round
Source
report of J.M.E. McTaggart (The Nature of Existence vol.2 [1927]) by Robin Le Poidevin - Travels in Four Dimensions 08 'McTaggart's'
Book Reference
Le Poidevin,Robin: 'Travels in Four Dimensions' [OUP 2003], p.130
A Reaction
[summarised] This has no ontological importance for McTaggart, since he thinks time is unreal either way. But giving the A-series priority because it reveals the present moment seems to nullify the B-series as incomplete.