I might think that he could count and bring (the omer), and harvest whenever he wished; it is, therefore, written "when the sickle begins (to harvest) the standing corn." I might think that he could harvest at night and count at night and bring (the omer) at night; it is, therefore, written (Ibid.) "from the day that you bring (the omer)."
I might think that he could harvest in the daytime and count in the daytime and bring (the omer) in the daytime; it is, therefore, written (Ibid.) "Seven complete Sabbaths shall they be," (the counting until Shavuoth not being complete if the first day is counted in the daytime.) When are they complete? When you begin in the evening.