find closest date in sorted array of dates if required date is missing

[tiliavirga]

So actual problem is that I have to find a date with minimum possible readings.

Okay, since the dates are sorted you can find the lower limit by way of a binary search. With 12000 dates it will be 14 accesses (2^14 = 16384) at the most.

Then dates follow in order from lower to upper limit so the number of additional accesses will be the same as the number of dates inside the range.