Jizz: well done for checking up on them. If more peeps did this we wouldn't have so much stealing!
MR: not quite. the HDP is calculated and paid based on your Home Base local time, regardless of the time of departure or arrival. So, if you are Aus based, leaving HKG at 2330 on the last day of the month, none of that 30 mins counts towards that month, but all of it counts towards the next month's HDP.