Seasonal changes are good (also your idea of the uniforms becoming more and more tattered is excellent)-—
but for urban characters the changes in season are not so important.
Although the major holidays mark time — Halloween, Christmas especially.
If you are looking for the passage of a lot of time (like years) then maybe a little bit of current history.
For example: Clinton gave way to the son of the man he defeated. Or: In the intervening years, the Towers fell, etc. Or finally, Henry Fonda won his Oscar.
You could do it that way as well.