The rabbis currently do not have the authority to declare a leap year because there is no Sanhedrin.
The current calendar was set up by Hillel II and is completely fixed by calculation. Indeed, even though the current calendar does have an error of one day in 216 years, we cannot fix that until the Mashiach comes and a new Sanhedrin appointed.
As we can see:
As a remedy, the Sanhedrin instituted the Hillel II calendar in 4119 AM -359 CE.
In one sense it preserved uniformity among Jewish people
abroad, but in another way it severed ties between Israel and the
The calendar itself, in general, is quite accurate, averaging 29.5
days per month and 365.2468 days per year (compared to the average
solar year length of 365.2422). It allows for adding and subtracting a
day in consecutive years as needed, so as to avoid waving the lulav on
Shabbat, or landing Yom Kippur adjacent to a Shabbat.
Implementing the Hillel II calendar was the Sanhedrin’s last big
decision before disbanding. Whenever the Hillel II calendar will be
decommissioned in the future to return to sighting and declaring the
New Moon days, it would have to be at the authority of a reconvened