Orthodox believe it is December 25th. They just use a different calendar.
As for Armenians… Theophany and Christmas were originally one feast on the 6th of January. Armenia simply kept that tradition, while the re-calculation of Christ's birth to be exactly 9 months after the Annunciation led to Christmas being moved to December 25 in the rest of Christendom.
In all cases it's not a big deal - the feast of feasts is Easter, and that's the date that matters the most.