I like the hypothesis that it represents a shiplog entry rather than a form of interstellar calender, not that it's a valid hypothesis.... Gene originally used it as a way to avoid mentioning what year Star Trek takes place in. It wasn't until much later that someone tried to use it as an actual calender.
Use this calculator, it uses the same conversion as in game. Basically the in game year is 400 years ahead of the date you enter and it is then converted to stardate format. So for 2409, enter a date in 2009.
Um... the STO one doesn't do dates in 2009... only 2010-2013.
sample date: 2410,1,1,12:00
StevePugh: 87001.36(might be assuming 12 pm)
STOacademy: 87607.97(this one is assuming it was 12 am)
Well, whatever the case, a lot of the game is set in 2409, but as has been mentioned every character is in a different time. That's the unfortunate nature of an MMO.
That being said, one easy way of setting stardates is to just use the current date and convert that to the in-game stardate. That's what I did in my missions, at first. Although now it's a bit behind the times.
I think using the STO Academy calculator makes the most sense since it's in-line with the method used in the game. In addition, all of the Stardate calculations were rather contrived. TNG just increased them with each season, for example. TOS had basically no coherent system as far as I know.