If I understood your question correctly, which I probably didn't. You're trying to convert a inputted amount of days, and have it sort itself out?
function serverCmdInputDays(%client, %days)
{
%y = (%days / 30) / 12;
%m = (%days / 30) % 12;
%d = %days % 30;
messageclient(%client,'', %m @ "/" @ %d @ "/" @ %y);
}