Starting Shavuot early (by R. Enkin at Hirhurim)

In this Hirhurim post, Rabbi Ari Enkin presents various opinions as to whether any aspect of Shavuot observance may be performed before sundown. This post of his is of interest to me because I think our current rabbi, who doesn't wait for sundown to start Arvit/Maariv (Evening Service), doesn't agree with a former rabbi, who insisted on waiting until sundown. Personally, I go with the former rabbi, on the grounds that we haven't counted seven full weeks and/or 50 days (the verse on which Sefirat HaOmer/the Counting of the Omer is based isn't clear) until after sundown. But it's good to know that my current rabbi has a halachic leg to stand on in his disagreement.