I posted yesterday and the post date said the 26th and it was the 25th. I went and checked the web and found out I was a -6 not a +6 from GMT.
If you post late in the afternoon it may show the next day- go to your profile page and change the timezone to a minus instead of a plus.
I know there are a few that need to look at this as I seen a few post yesterday dated today. Here is the site I used. Just click on your state