Actually the terms should be (to be highly technically correct I suppose)
CenturyDateTime2ELdate and CenturyDateTime2ELtime
inverted
ELdate2CenturyDateTime and ELtime2CenturyDateTime
The fact that it crosses two centuries would not impact the use of Century in the name (correcting the miss use of Julian).
Sort of related to your comments, Yes I am not sure why Julian YY### format would still be used when these types of century calculations are available now.
I think if these names were used I would have understood this whole area almost immediately since I did those school projects many years ago. Maybe I will write a wrapper function to call these julian reserve words so I start using the correct names. It might be handy when I get into my 90s . . . when and the mind slows down even more and am making billions on every trade