Since the inquiry is about tense, it should be pointed out that the
construction is generally followed by present perfect.  (In the past
four years, we have built twelve homes.)    As such, it conveys past
action that is continuing into the present.  (Or a time period that is
not over.) It often comes in sentence opening position, what functional
grammar calls MARKED THEME.  (Not the grammatical subject, but a
stepping off point for the message structure of the statement.)  I hope
that helps.

     Craig Hancock, University at Albany