Your math is fine but rather than the halvening being really based on the calendar, its based on the number of blocks. These blocks occur approximately every ten minutes. So rather than every 4-years it is every 210,000 blocks. This should be every 1,458 days and eight hours (which is very close to 4-years) if the blocks take an average of exactly ten minutes a block. Due to randomness it shouldn't take that amount of time as people mine faster when they the industry mines more or mine slower when miners quit.

The schedule is based on blocks. You could do the same with Hive. The Hive blockchain is supposed to take three seconds for each block. And since bitcoin's blocks are 600 seconds, you could make the halvening every 42,000,000 blocks (200*210,000) this would approximate bitcoin's design of issuance of every 1,458 and a third days.

Bitcion doesn't really get that close to it's schedule though. People mine it too fast and the difficuty doesn't keep up. For the first halvening, it came two months early ahead of the 1458 days. It should have been in January of 2013 but instead happened in November of 2012. Then after that it was even more so with the halvening happening in July of 2016 rather than November. Five months early! Finally the last halvening of May last year was two months ahead of schedule. It also means there are over 5% more bitcoins today mined than there was in the master plan. The measured average blocktime of BTC calculated from the table you pointed to is 9 minutes 29 seconds per block.

To really get what bitcoin has been so far you should have halve every 39,802,000 blocks.