pandas.merge: match the nearest time stamp >= the series of timestamps
Pandas now has the function merge_asof, doing exactly what was described in the accepted answer.
I have two dataframes, both of which contain an irregularly spaced, millisecond resolution timestamp column. My goal here is to match up the rows so that for each matched row, 1) the first time stamp is always smaller or equal to the second timestamp, and 2) the matched timestamps are the closest for all pairs of timestamps satisfying 1). 
Is there any way to do this with pandas.merge?