Forty days of Yamim Nora'im (Days of Awe)
The 29-day month of Elul
The month of Elul is the month of Hesbon Nefesh.
One or two days of Rosh Hashanah
Rosh Hashanah is observed for two days outside of Eretz Israel.
The ten days of Teshuvah begin with Rosh Hashanah and conclude with Yom Kippur, inclusively. In other words: the beginning of Rosh Hashanah is also the first day of the ten days of Teshuva; and Yom Kippur is the tenth and last of the ten days.
The day of Yom Kippur comes exactly ten days after Rosh Hashanah starts. This concludes the Days of Awe.