Translation:We will have lunch together at twelve-thirty.
How do i distinguish between time and date? For example will eat lunch at 12:00 or will eat lunch on the twelvth (of the month).
Tio okazis la dekan. = Tio okazis en la deka tago de la monato.
Tio okazis je la deka. = Tio okazis je la deka horo de la tago
Are those rules a bit arbitrary or am I missing something? Aren't the twelfth of the month and twelve o' clock both equally specific times? Why does only one take the -n suffix?
https://en.wikipedia.org/wiki/Esperanto_grammar#Prepositions "in Esperanto all prepositions govern the nominative. The only exception is when there are two or more prepositions and one is replaced by the accusative."
Happily there have been other responses for you.
I want to add that every rule in a conlang is arbitrary, but there is nothing inherently bad or good about that fact.
I love that Duo accepts "We will lunch together at half twelve." It sounds so aristocratic.
Duo and assumes all English speakers call the main meals breakfast, lunch, and dinner, even though this is not the case in all areas, or even in all Esperanto learning books.
How about "We will eat lunch together at twelve-thirty"? Duo counted it wrong, but it's not very different from the correct translation.