Re: Greek plosives

staving veritosproject:
>I was studying Greek's orthography a bit, and I realised that although
>it seems to have the letters for voiced stops (beta, gamma, delta) it
>seems to be using the combination (nasal+unvoiced plosive), as in "nt"
>for "d".  Why is it doing this, or is it just for transliterated
>words?
Classical Greek voiced stops underwent sound change to fricatives, hence
beta, gamma and delta are pronounced [v] [G] and [D] in Modern Greek.

Does anyone know exactly when this took place?

Pete