365 days vs 360 days


can someone summarize when we have to use 360 days and when 365 days? When we annualize a weekly or monthly return we have to use 365 days but I think working with any financial instrument we have to use 360 days in our calculations, is that correct?



From what I’ve seen is that you’re correct. I’ve only encountered 365 day count when calculating EAR. 30/360 or actual/360 in cases like interest payments, option pay-offs etc.

360 T - Bills and I think Munis but not sure for those. You feel free to use 365 day count everywhere else. I think also LIBOR is quoted on 360 basis but it will prove Mr. Magician.