"They had to travel for a whole day before they reached the mountain."
Translation:Ili devis vojaĝi dum tuta tago antaŭ ol ili atingis la monton .
it seems that 'ol'/than is required here. Why? I seem to be able to translate it easily without what seems to be the unneeded 'ol'.
"Antaŭ" is used before expressions of time or action (usually nouns). "Antaŭ ol" is used before verbs and phrases which contain verbs.