Undefined date header.

After many, many tries to solve the problem of the undefined date in the end I managed to overcome it. It's so simple.

Settings> Formatting> change Date Header Format & Timestamp Format so they match > Save and then View Blog.

For example, Date header: Month, Day, Year. Timestamp Format also: Month, Day, Year. Don't forget to save it before viewing your blog. I found this from a web. Hopefully whoever have this type of problem can work it out too. All the best! :)