The use determines the approximation If the mean tropical year is under discussion then an approximation of  days might be appropriate But the decimal year is under discussion here and astronomical epochs within a year have used the Julian year since  Before  the Besselian year a kind of mean tropical year was used Epochs wihin both years were specified as decimal fractions of a year These epochs are used whenever some variable changes over the course of a year requiring the position within a year to be specified where  means the beginning of that year whereas  means the middle of that year The change was necessary because the mean tropical year changes from yeartoyear and even within a particular year making calulations using it inherently imprecise The Julian year was chosen because it is a simple approximation of a calendar year even simpler than an average Gregorian year of  days changing to an Egyptian year of exactly  days would have been too extreme Furthermore the day within the Julian year contains exactly  SI seconds avoiding the variable length of the day over various time scales from days to millennia again a change needed for precision The days within the former Besselian year were variable mean solar days instead of uniform SI days now used