SpaceX has in particular been notorious about delaying their recent flights. The advent of the Falcon 9 v1.1 was about a year late, as you can see from looking at some of the mission dates in the history on SpaceFlight Now. For instance, the upcoming CRS mission shows that the following have been listed as it's launch date:
April 6, Sept. 30, Nov. 11, Dec. 9, Jan. 15, Feb. 11, Feb. 22, March 1 and March 16. [March 14]
SpaceFlightNow is a fantastic resource, even many in the industry use it to keep track of things.
As for the launch of the Falcon 9 heavy, I wouldn't put any serious money towards watching it happening until it's a month away. Even then, don't be surprised if it doesn't launch for a few weeks after the scheduled date. Stuff happens in the space industry.