for the doc :
http://docs.splunk.com/Documentation/Splunk/6.0.1/SearchReference/CommonStatsFunctions
range(X) This function returns the difference between the max and min values of the field X ONLY IF the value of X are numeric. chart, stats, timechart, sparkline()
Maybe a rounding issue, or a missing value.
To verify that your values are actually not null, not identical, or not missing, please try :
FILE="*" | stats values(timestamp) min(timestamp) AS mintimestamp max(timestamp) AS maxtimestamp range(timestamp) as durationms by FILE, FileSize
| eval manualrange=maxtimestamp=mintimestamp
| eval duration=durationms/1000
| eval manualduration=manualrange/1000
| eval manualduration_round=round(manualrange/1000,10000000000)
... View more