Source: Orthodox England
Isn't the true Orthodox calendar the old calendar?
Your question is a provocative one, and I would therefore take advantage of it to dispel a myth.
All Orthodox Christians (with the isolated and highly controversial exceptions of a handful of parishes under the Moscow Patriarchate in Holland and about twenty or so parishes in Finland which some consider to be under anathema) celebrate Easter and the feasts of the Easter cycle on the same Orthodox (so-called 'old') calendar. This is the calendar which has always been used by the whole Orthodox Church (and also Roman Catholicism until the end of the sixteenth century).
True, over the last few decades a minority of Orthodox (about 25% of the total) have moved on to the Roman Catholic ('new') calendar for the fixed feasts like Christmas. They have done this mainly because they find it convenient. Indeed, for some Anglicans and others converted to Orthodox Christianity, this use of the Roman Catholic calendar might be a useful and convenient bridge for them as they approach Orthodoxy.
There may be a real danger in the use by some Orthodox of the Roman Catholic calendar for the fixed feasts, not on account of this use in itself, but of the development of 'New Calendarism'. By this phrase I mean the refusal of some to concelebrate with those who use the age-old Orthodox calendar, contemptuously called 'the old calendar'. Of course, I would certainly agree that 'Old Calendarism', the reaction to the often violent and intolerant persecutions of New Calendarism, can also be dangerous. Just as New Calendarists are often fanatically opposed to any shred of honest Orthodox Tradition, so some Old Calendarists refuse to concelebrate with those who use the Roman Catholic calendar for the fixed feasts. It seems to me that in the calendar question we should avoid extreme positions. Thus although a change to the use of the Roman Catholic calendar for the fixed feasts is unthinkable for the vast majority of Orthodox, we who are in the majority should be tolerant of those who find it pastorally helpful to use the Roman Catholic calendar for the fixed feasts; just as those who use the Roman Catholic or 'new' calendar for the fixed feasts should be tolerant of those who are more traditional. Certainly the idea of refusing to concelebrate with new-calendar Orthodox is quite absurd. Such personalities as Metropolitan Antony (Khrapovitsky) or St John the Wonderworker had no qualms about the use of the new calendar for the fixed feasts, when pastorally necessary.
My answer to your question is: the true calendar is the calendar of Truth and Love.