Today (23 March 2020) Israel's Supreme Court rejected a petition to declare this year a leap year so Pessah can be postponed for a month, and the seders don't have to occur during a lockdown (hopefully!).
I don't know why the petitioners went to a secular court, but do the rabbis have the authority to declare a leap year under the circumstances?
There are two problems, both are discussed (among others) in Sanhedrin 11a. The first is that you need a Sanhedrin with a Nasi to intercalate a year. The second is that there are certain reasons for intercalating a year (e.g. damaged roads and bridges to Jerusalem, ripening of the grain and fruits). Actually, one of the reasons for introducing a calculated calendar was to avoid the dependence on the Sanhedrin, which was vulnerable of being disbanded by the oppressors. As Rambam says (Hilkhot Kiddush haChodesh 5:2–3):
וּבִזְמַן שֶׁאֵין שָׁם סַנְהֶדְרִין קוֹבְעִין עַל פִּי הַחֶשְׁבּוֹן הַזֶּה שֶׁאָנוּ מְחַשְּׁבִין בּוֹ הַיּוֹם וְאֵין נִזְקָקִין לִרְאִיָּה. (...) וּמֵאֵימָתַי הִתְחִילוּ כָּל יִשְׂרָאֵל לְחַשֵּׁב בְּחֶשְׁבּוֹן זֶה. מִסּוֹף חַכְמֵי הַגְּמָרָא בְּעֵת שֶׁחָרְבָה אֶרֶץ יִשְׂרָאֵל וְלֹא נִשְׁאַר שָׁם בֵּית דִּין קָבוּעַ.
When there is no Sanhedrin, the monthly calendar is established according to the fixed calendar that we follow now, and the sighting of the moon is of no consequence. (...) When did the entire Jewish people begin using this calendar? At the conclusion of the Talmudic period, when Eretz Yisrael was in ruin, and an established court no longer remained there.
So current rabbis would only have authority to intercalate a year if they formed a new Sanhedrin and the aforementioned criteria from the Talmud would hold.
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 Diaspora.
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 Sanhedrin.
