Have this ever happened to you? Actually this is simple problem. Template designer usually set this date header not display considering its appearance. How to solve this problem? There's two alternative solving problem.
1. Check HTML Code
First, you have to check html code, as I said, the author could be set date display none. Go to EDIT HTML. If there's code below
margin: 1.5em 0 .5em;
Delete display: none;
And then Save. Preview it and date will appear.
2. Change Timestamp Format Setting
Even the author set not to display date header, usually they still set to display timestamp. Actually we can this tricky setting.
Go to Setting > Formatting > Timestamp Format
There's many choice of timestamp format setting. Choose timestamp that show date, voila, your date will appear.