Robert. You’ll find quite a lot of different date and time formats are used in astronomy. The decimal date format is quite common for things that vary (such as outbursting comets) since it allows fairly quick mental calculation of the time between events. The next step is to get rid of years, months and days altogether and quote in terms of Julian Date. This is used a lot too but it is more difficult for the average human to interpret. If we arranged to meet at a pub at JD 2460254.333 most people (except some variable star observers) would have to go and look up a more familiar date before leaving home.
The point made by Paul Leyland up the thread is important too. In science, when we talk about quantities, we don’t want to imply more precision than is actually present. If you took it literally, July 20.82 would be 19:40:48 but we clearly don’t know the time of the outburst to that precision. The use of two decimal places in this context is used to indicate the level of precision that we think we have, i.e. around one hundredth of a day or around 15 minutes.