We can translate the sentence <em>hace tres meses que conozco a Juan </em>into <em>I've known Juan for three months. </em>So the group of words hace tres meses comes from a common formula in Spanish:
<em>Hace + number + días/meses/años</em>
that is used to tell you something has happened from the past until now. If the number is uno (one), <em>días/meses/años </em><em>(plural form)</em><em> </em>changes to <em>día/mes/año </em><em>(singular form)</em>
