You can do this pretty easily in Excel - as I recall:-)
And presumably in Libre Office.
Just tried excel and by typing the dates in as
a1 -> "23 may 2016"
b1 -> "23 june 2016"
Excel recognised the entries as dates.
and in an adjacent cell "=b1-a1"
poof - presto done.
Cells were automatically sorted out with correct everything to display days between.
23-May-16 23-May-17 365