If you hover your mouse over an underlined bit of text, or the red circle with the X to the left of the line it will pop-up the error.
If you start typing a line and press CTRL-SPACE it will pop-up a dialog with all valid ways you can complete the line.
For example typing Garage_Door<CTRL><SPACE>
will pop-up a dialog with all the methods that are part of the Garage_Door Item. You can arrow down through the list and select the method you want and it will auto-complete the line.
Here is some code I use to determine whether the current time is between two time periods (scroll down to the “Get time period for right now” rule):
It would be best to use the latest version of Designer as there are minor changes necessary to keep the syntax highlighting up to date with changes in the server.