Making Our Days Count

There is a widespread custom that men stay up the entire Shavuos night and learn Torah. Of course the entire night really isn’t that long. This is partly due to another custom: we do not accept the holiday of Shavuos early. Within a few blocks of my house there are at least four minyanim that daven Kabbalos Shabbos before sunset, each one starting at a diffent time. Why is the same leniency not employed on Shavuos?
The pasuk states, “Sheva Shabbasos temimos tihiyena”—They shall be seven complete weeks.” This a special mitzvah that the seven weeks of the Omer that we count must be complete. Further, Tosfos (Menachos 66a d”h Zecher) quotes the Hilchos Gedolos that if one forgot to count one day of Sefiras HaOmer he can no longer continue counting with a berachah. In his opinion, that would be a violation of “temimos”—being complete. The individual cannot complete the count of 49 days, if he missed one day.
Another facet of temimos is mentioned in the Gemara. The Gemara (Menachos 66a) states that the significance of temimos is that we start counting the Omer at night. If we waited until the day, one night would be lacking from the first week. The Ran in Pesachim quotes an opinion that it is preferable that the counting of the Omer on the first day (the second day of Pesach) should be done at the earliest possible time of the night. One could argue that even a slight delay would cause the seven weeks not to be complete. As a matter of halachah, counting as early as possible may be prefered, but one would certainly fulfill the mitzvah of counting at any time on that first night.