Language::formatDuration formats date intervals in seconds to human-readable strings like "5 days, 2 hours and 8 seconds".
It supports a number of time intervals like "years", "weeks", 'days" but not "months". I suspect this is complicated because months are irregular but from the user's perspective a string like "2 months and 4 days" [ago] makes perfect sense even though it can be ambiguous.
 On March 31, "1 month ago" refers to when exactly?
This change would be useful on the newcomer homepage, which is a feature to help newcomers get oriented when they first create their account, so they can figure out how to start editing quickly. In particular, in the parts of the homepage that say how the long the user has had their account, and that say how long since their mentor has been active.