Q. I always wondered, since on Shavuos one is to start the Yom Tov only at the time of when the stars show and true night begins, so the Seven Weeks will be complete. If so how does one comply with the mitzva of Tosefes Yom Tov or adding some time before the beginning of Yom Tov, that applies at least on a Rabbinical level also on Yomim Tovim?

A. Indeed Mishna Berura (494: 1) rules that we start Maariv on Shavuos night by Tzeis Hakochovim to keep the complete 49 days of the Sefira. However, some Poskim maintain that it only applies to reciting Kiddush, yet one may daven Maariv earlier. (Sheloh and Mogen Avrohom ibid.).

Tosefes Yom Tov similar to Tosefes Shabbos can be complied with just accepting not to do any prohibited Melochos without receiving the complete Kedusha and holiness of the day.
Horav Shlomo Miller’s Shlit’a opinion is similar

Rabbi A. Bartfeld as revised by Horav Shlomo Miller, Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a