2.15b15 and probably some versions before (it's also in the demo):
selected period not respected for graphs
dashboard Visits over time widget
=> in wide main graphs of this pages and also in small graphs
screenshots taken from demo:
The huge question is: is it respected for data shown in tables??
an other place where the problem appears:
screenshot taken from demo
we can reproduce it with different browsers in piwik demo (screenshots above taken from) and our own installation (2.15b15)
the STRANGE thing: in a friends installation also on 2.15b15 everything is fine with same browser types!!
the mentioned friend tested demo.piwik.org with the same browser with which in his own installation everything shows up fine:
he could reproduce the problems in the demo
=> so it seems not to be a browser/plugin issue
I can concur with Matt, even if it often bugged me it always (back in 1.11) worked that way.