Learn Finnish: Juna lähtee kymmenen minuuttia vailla kaksi. - The train leaves at ten minutes to two.

It translates to "to" when telling time (as in "ten to two"). Its literal meaning is "without" or "lacking". You will also frequently hear the variation vaille used in exactly the same way.

Why is the word for minute written as minuuttia instead of minuutti?

In Finnish, any number greater than one requires the word following it to be in the partitive case. Because we have the number kymmenen (ten), the basic word minuutti takes the partitive ending and becomes minuuttia.

If the train leaves "at" ten to two, why isn't there a word or ending for "at" attached to the time?

For exact, full hours, you add the ending -lta or -ltä to mean "at" (like kahdelta for "at two"). However, when you give a more complex time using vailla (to) or yli (past), you don't need any special "at" ending. The time phrase is just stated as it is.

Is it necessary to include the word minuuttia every time?

Not at all! Just like in spoken English, Finns usually drop the word for "minutes". It is very natural and common to shorten this to just kymmenen vailla kaksi (ten to two).