It appears that sometimes prices are permitted when the start and end-date overlap, and other times it does not.
It's not clear to me what constitutes overlapping. If they're internally datetimes, then this might be permitted:
price 1: 25/1/2016 -> 20/2/2016
price 2: 20/2/2016 -> no end
... and the system seems to allow this most of the time. Please see attached screen shots.
One of them shows the system rejecting overlapping date ranges, and in the other it shows the system accepting overlapping date ranges.
Ubuntu 14.04.2 LTS Server + MySQL + Tomcat7