import groovy.time.TimeCategory

random = new Random()

Date randomDate(Range<Date> range) {
   def res = range.from + random.nextInt(range.to - range.from + 1)
      use( TimeCategory ) {
          res = res + random.nextInt(24).hours + random.nextInt(60).minutes + random.nextInt(60).seconds
      }
      res
}


def start = Date.parse('yyyy-MM-dd mm:ss', '2015-01-01 00:00')
def end   = Date.parse('yyyy-MM-dd mm:ss', '2017-12-31 23:59')
println randomDate(start..end)