Your issue is that an underscore is part of valid variable name, hence when you pass the string to tar it looks like:
As you can see from the colours, you have no variable called $DATE_Time_
When you have additional parts that need to be attached though, bash has the curly brace enclosure to assist with sectioning off the variable names.
So you can use:
Now this will only look for $DATE and will append _Time_.
The curly brace enclosure can actually be used around all variables all the time. It does make your code cleaner however to only use them when required.