If you use use the Salut et Fraternité iOS app or have the "Romme" option set in the Calendrier app, or simply follow the French Republican Calendar using Romme's proposed reform, then you can celebrate today, 22 September 2010 (55461), as the first day of the new year, CCXIX (219). According to Romme's proposal, a leap day is added to the end of years divisible by four. Brumaire, maker of S&F, adds the leap day to the end of the previous year. Since 219 is neither one of these, there is no leap day, and therefore today is a new year.
However, if you use the original rule, which was the only one ever legally in effect, the new year starts at midnight on the day when the autumnal equinox occurs in Paris. The equinox occurs tonight at 20:09 my time, which is 03:09 GMT (55462.13111). France is currently two hours ahead of GMT, so that would be 5 h 9 in Paris, but actually the law specifies temps vrai de Paris (Paris apparent time), which today is about 16 or 17 minutes ahead of Greenwich, so the equinox is around 23 sept. 2010 à 3 h 25 t.v.P., or 142 decimal minutes past local apparent midnight.
Anyway, that means that tomorrow, 23 September (55462), is without doubt the first day of the new year, which means that today is a leap day, the sixth (sextile) complementary day, the Day of Revolution.
It is already tomorrow (summer time) in the eastern hemisphere, so whichever day you celebrate, bonne année !
Sextidi 6 complémentaire an CCXVIII à 9hd 86md t.m.P. au
Primidi 1er Vendémiaire an CCXIX à 9hd 86md t.m.P.