Joining time fields?

Hi all,

 

I wonder if someone could help me out please. I've got two tables, both with a TIMESTAMP field. I'd like to do a left join with them. I'd like to take the date_time from table A, and join it with the nearest date_time from table B. Whether the time from B is before or after the time in A doesn't matter, I just want the closest time. I started with the below query, but it only gets me the column from table B if the time stamp exactly matches which is clearly correct. I'm sure that this should be quite easy but I can't figure it out...!