http://revach.net/article.php?id=282

Section: Halacha   Category: Tshuvos
Sridei Eish: Counting Sefiras Haomer After Shavuos

If someone continues counting the Omer after Shavu'os it is not a transgression of Bal Tosif. The reason is that the Torah didn't just command us to count forty nine days.  Rather the Torah commanded us to count a specific forty nine days - the days between Pesach and Shavu'os. Consequently counting after Shavu'os has no connection to the Mitzvah at all.

This concept helps explain an enigmatic Ramban that says that Sefiras Haomer is not a time bound Mitzvas Aseh. According to the above logic Sefiras Haomer is not simply a Mitzvah that is done at a specific time, rather the time that the Mitzvah is done is a definition of the Mitzvah. We must count the forty nine days between Pesach and Shavu'os. It is the time that defined the Mitzvah, not just a Mitzvah that is done at a specific time. (Sridei Eish 2:90)