Bug 8486 Correct calculation of days_between`

Bug 8486 Correct calculation of days_between`

This patch backports the following patches
from master to 3.8.x:

commit d786d7c7b6c2894cca6231cae3deee9c9d43120b
Author: Colin Campbell <colin.campbell@ptfs-europe.com>
Date:   Tue Jul 24 15:16:10 2012 +0100

    Bug 8486 Correct calculation of days_between

    There were errors in the calculation used for days_between
    which caused incorrect values to be returned
    Added tests to validate calculation

    NB Tests still need to be provided for the hourly
    calculations

commit 8379a9781dbf16bcb53b55b20f15dcd55ff948Oc2
Author: Colin Campbell <colin.campbell@ptfs-europe.com>
Date:   Wed Jul 25 10:55:56 2012 +0100

    Bug 8486 Merge Calendar test files

    Moves the tests previously in t/Kalendar.t to t/Calendar.t
    and removes the old file.
    Note the existing minimal test of C4/Calendar still called in
    t/Calendar

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>