Recently I got a lot of cr*p from a trader when discussing some calculations concerning the zero curve, he wouldn't be willing to explain why I made a mistake in my calculations, so let's see if you can shed some light.
Assume I have a yield curve (in ACT/365) built using depo rates up to 12M and then swap rates from 2Y on. For the purpose of checking that the yield curve had been built properly, the trader asked me to imply the original 2Y Swap rate market raw input (quoted in 30/360 ) from the 1Y and 2Y point of the zero curve. This was easy, I did all my calculations in ACT/365 but then I converted my result into 30/360 by multiplying by the year fraction between spot and 2Y in 30/360 and dividing by the same time bracket but in ACT/365. The trader claimed this was wrong and that I should have done all my calculations in 30/360 from the beginning.
Why was my rate conversion conceptually wrong as he claims?