Use 1970-01-01 00:00:00 for time that can't be handled by mktime()
Use 1970-01-01 00:00:00 for time that can't be handled by mktime()

refs #1476

yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,naoa/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga,mroonga/mroonga