The message uses concatenation to build complete sentence. This is a practice discouraged by the MediaWiki guidelines, and unfortunately in this case it goes beyond just theory: it adds a message that has text in nominative case, whilst for the correct uk sentence one would need to write the time period in genitive.
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I suppose this is yet another instance of T69959.
As a personal opinion, I can't see why the "ago" mechanism exists and I'm strongly in favor of displaying just the timestamp as-is, as per standard MW practise, and letting the end-user calculate the difference in their head.