‘Human-readable’ time short formats conventions?

I’m trying to show a so-called human-readable time format, but not sure about the best way to display short versions of minutes vs months.

You have:

• 1 second ago → 1s
• 1 minute ago → 1m
• 1 day ago → 1d
• 1 week ago → 1w
• 1 month ago → ???
• 1 year ago → 1y

Also, probably it’s only me, but I think I rarely see weeks, e.g. is it preferable to show 7d vs 1w?