Stardates Explained - 20170918.1200

You don't understand the stardates?
Take, for example, 20170918.1200.

Let me stick some spaces in some helpful places:
2017 09 18 . 12 00

That better?

20170918 means 18th Sep 2017, and
1200 is in 24-hour clock format, IE 12:00 midday

Stardates in the Star Trek universe work in an entirely different way. Wikipedia has a good explanation on how they work.

Events on this stardate