When давнћ is used with the past tense (of either aspect) it means 'long ago'. Compare this with давнћ plus non-past Imperfective (i.e. present time):
Давнћ + Past: 'long ago'
Ћн давнћ здљсь учќлся. He studied here a long time ago.
Јто бІло давнћ. That was long ago.
Давнћ + Non-past Imperfective: 'for a long time now'
Ї давнћ её знђю. I've known her a long time.
Ћн ужљ давнћ здљсь џчится. He's been studying here a long time now (Lit. 'already').
Recall that дћлго means 'for a long time' and can be used with any Imperfective tense. Unlike давнћ, it expresses simple duration, not necessarily an event lasting up to the present.
Онђ вчерђ дћлго рабћтала в лаборатћрии. Yesterday she worked for a long time in the lab.
Онђ и зђвтра бџдет дћлго рабћтать. She'll work long hours tomorrow, too.