dashboard: fix Date arithmetic calculations
dashboard: fix Date arithmetic calculations

Bis repetita placent...

The fix in aa4d780983135f9be41705598f2aa9749a2ddeed wasn't correct.

This also forces the default end date to be now() as most graphs
don't look into the future.

Signed-off-by: Pierre-Alexandre Meyer <ff019a5748a52b5641624af88a54a2f0e46a9fb5@ning.com>

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