I don't see it as an unknown time. Its unknown only for unbelievers, the rest can know the approximate month but not the day or the hour.
Quite often that day is described as a thief for unbelievers, but not a thief for believers: 1 Thess 5:
Now, brothers and sisters, about times and dates we do not need to write to you, for you know very well that the day of the Lord will come like a thief in the night. While people are saying, “Peace and safety,” destruction will come on them suddenly, as labor pains on a pregnant woman, and they will not escape. But you, brothers and sisters, are not in darkness so that this day should surprise you like a thief. So are you trying to say that the month will be know?