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

We cannot simply add + 3 to any month number...

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

killbill/killbill-analytics-plugin,kisdead/killbill-analytics-plugin,kevinpostlewaite/killbill-analytics-plugin,daliwei/killbill-analytics-plugin,kisdead/killbill-analytics-plugin,killbill/killbill-analytics-plugin,daliwei/killbill-analytics-plugin,daliwei/killbill-analytics-plugin,kevinpostlewaite/killbill-analytics-plugin,kisdead/killbill-analytics-plugin,daliwei/killbill-analytics-plugin,kisdead/killbill-analytics-plugin,kevinpostlewaite/killbill-analytics-plugin,kevinpostlewaite/killbill-analytics-plugin