Format?
Display,value
2018-06-11,1528642800
2018-06-10,1528556400
First and last "_time"?
If you only search
(your search)|stats max(_time) as max_time,min(_time) as min_time
If you want to add it as a field
(your search)|eventstats max(_time) as max_time,min(_time) as min_time