Tosefes Shabbos and Yom Kippur-Adding on to the holiness of Yom Kippur:
It is a Biblical command to add on to the time of Yom Kippur, both in its beginning and end. This means that one is to begin to keep all the laws of Yom Kippur prior its official entrance by sunset and after its official leave after nightfall the next day. This law likewise applies for Shabbos and all Holidays. Due to the above Yom Kippur usually extends for up to 26 hours.
How much time does one need to add on to keeping Yom Kippur? There is no minimum amount of time that one needs to add to the entrance and leave of Yom Kippur. Thus even if one adds one moment he has fulfilled the Mitzvah. [However some learn based on the Siddur that one must add 4 minutes to the entrance and leave of Yom Kippur, Shabbos and Yom Tov.]
Practically from when should one begin Yom Kippur? All the prohibitions apply starting from sunset of Erev Yom Kippur. When one stops doing Melacha and begins the oppressions from sunset he has also fulfilled the Mitzvah of Tosefes Yom Kippur.
 In today’s Jewish calendars this addition is already included in whichever time they list for the beginning and end of Yom Kippur. However one who is going in accordance to sunset would need to add some time before sunset, and sometime after night by the leave of Yom Kippur.
 Including not doing Melacha and already keeping the five oppressions such as not eating and drinking leather shoes, etc. [608/1]
 However the source for this law is learnt from a verse speaking about Yom Kippur.
Opinion of Rambam: The Rambam in Mishneh Torah rules that 1)The adding of time only applies to Yom Kippur and not to Shabbos and Yom Tov 2) Even by Yom Kippur one only needs to add in his fasting and the like, however to do work is permitted until the official beginning of Yom Kippur, and after the official end of Yom Kippur.
 Hayom Yom
 Ketzos Hashulchan 93 footnote 1
 See 611/1 and 608/1-2 Regarding that all the laws both of fasting and oppression begin from the time of Tosefes Yom Kippur.
In Seder Hachnasos Shabbos in Siddur Admur explains that in truth the real sunset is about 4 minutes later than the visible sunset. The Ketzos Hashulchan [93 footnote 1] learns from this that according to Admur one fulfills the Mitzvah of Tosefes by stopping Melacha from sunset.