There's a doubt in the Gemara when the new day starts, at sunset or at three stars (Ok, and there's Rabbi Yossi's opinion). When it comes to Shabbos (or any other biblical time), we have to be strict ...
Is the dictum of safek dioraisa l'chumrah (and its inverse safek d'rabanan l'kulah) a principle from the Torah, or a din d'rabanan? How does this change how we react in these sefeikah situations?
Normally we say "sofek d'rabbanan l'kulo" and will be lenient in a case of a doubt that is only an issur d'rabbanan. I have heard that there are poskim who say this doesn't apply in the case when ...