Why the date is displayed as “undefined” or this is not show?

Probably because you don’t choose the right date format. The format should be changed from the panel and usually the correct types are:

1. Dashboard → Settings → Formatting → Date Header Format

In the new Blogger design: Dashboard → Settings → Language and formatting → Date Header Format

Or

2. Dashboard → Settings → Formatting → Timestamp Format

In the new Blogger design: Dashboard → Settings → Language and formatting → Timestamp Format

Read the instructions of your template located on the download page.