A join on _time can fail even if the epoch values match. The failure is caused if one epoch _time value has a numeric format with decimal places and the other does not.
If on left side, you have _time=1405206000
and on right side, you have _time=1405206000.000
the join will fail, even if on screen you will see the same value on both sides: 2014-07-13 00:00:00
If you first timestamp is always precise to the second and no more, here is how you can make your join safer:
eval time=round(time,0) on the right side.
Well yes, initially, I wanted to ask about my problem on joining on time... but then I figured it out, so I thought I would share the reply, even though I don't have a question anymore
Just edited the entire post to fit the Q&A format 🙂 Thanks for posting @manus. I think others will find this content useful