The summer internship piece includes several deadlines - final dates for successive stages in the selection process. Different things have to be done BEFORE these dates (applications submitted, English test taken and interview completed), and instead of "before" you can use the word "BY". What is tricky is that you can only use "by" for things which are DONE (completed, finished) before that date or time, so this is how to say "dovtedy to musí byť urobené".

If you want to say "dovtedy sa to musí robiť", then you have to use "UNTIL" or "TILL" (and remember that "till" and "until" are not positive or negative in English, but that "till" is the short form of "until), like this: Wait here until (till) I come back (don't say "will"!), or: Keep going straight on until (till) you reach the castle (Franz Kafka's most famous line), or: Dance till you drop (what with it being the ball season and all).