"The Taz (Teshuvos 600) discusses a scenario where a community did not have a shofar on Rosh Hashanah, which fell out on Friday. After the community accepted Shabbos early, a non-Jew brought them a shofar. Here’s the question: Do we still consider it daytime and thus the shofar can still be blown, or is it nighttime and there is no longer a mitzvah to blow shofar? The Taz gave two reasons for why they could blow shofar. First, accepting Shabbos is similar to making a neder, whereby if it was done b’taos (mistakenly) it is not valid. Since the community would not have accepted Shabbos if they knew that they would be receiving a shofar afterwards, the acceptance was done b’taos – and is not valid.This discussion is from an essay Counting The Previous Day’s Sefirah by Rabbi Raphael Fuchs, published May 9th, 2012 and updated May 10th, 2012 in JewishPress.com.
"Second, the Taz, quoting the Beis Yosef in the name of the Smag, says that in regard to calculating the eighth day for a bris milah we only look at whether it is actually day or night. It does not matter if one davened Ma’ariv or accepted Shabbos early; if it is still day the bris will be eight days from the day, not from the night. The Vilna Gaon explains that mitzvos that are not dependent on Shabbos, even if one accepts Shabbos early, are considered as if done during the day. Based on this the Taz ruled that they could blow shofar – even after accepting Shabbos."