Always format regular expressions as code. If you look closely at your post you'll note that it doesn't represent reality. All underscores have disappeared.
Unsurprisingly, capturing a time works the same way as capturing a date. I suggest the following expression for capturing the whole timestamp string (including the square brackets):
(?<timestamp>\[%{YEAR}_%{MONTHNUM}_%{MONTHDAY}\]\[%{HOUR}_%{MINUTE}_%{SECOND}\])
This string can then be fed to the date filter.