I have a spreadsheet that looks like this:

Time Conductivity
11:59:39 a.m. 33.4872776
11:59:39 a.m. 33.4202993
11:59:39 a.m. 6.3550293
11:59:39 a.m. 0.0567908
11:59:39 a.m. 32.0216573
11:59:39 a.m. 35.0264749
11:59:40 a.m. 35.0776638
11:59:40 a.m. 35.0743198
11:59:40 a.m. 35.0763977
11:59:40 a.m. 35.0793098
11:59:41 a.m. 35.0650993

It goes >72,000 columns long! until around 3:30:42 p.m and I need to average the conductivity to compress my data together. Ideally by seconds or minutes. The problem is that data is not even (eg, there is a different number of data for every second). What formula can I type in excell to do this?