Originally, there was little formality. Later in the movies the first two numbers roughly denote the year, with the following numbers further divide up the year. TNG adds a 4 to denote the 24th century, the second number is the season, and the next three represent what part of the year, and the decimal approximates the time. In the rebooted Trekm the four digit number is the year, followed by a decimal and a two digit "date"
An easy stardate is the last two digits of the year, the percent of the year passed, and the percent of the day
1200pm 8th june 2011 would be 1153.5 3AM March 17th 1995 would be 9525.25