A. (1) Check to be sure they’re from the same region. Depending on the longitude and latitude – timing differs. (2) There are several opinions in Jewish Law in terms of defining “dark” and “nightfall”. While the times we use are pretty much standard in the Jewish community, some may follow a different opinion. (3) If it is only off by a minute or so, that is because some round off to a more stricter time, while others leave it as is.