If you put the cursor on such a line you can:
0
to put the cursor in the first column.f
Space to put the cursor in the first blank.i
Enter Esc to split the line.l
to move the cursor one line up..!date +\%Y\%m\%d
to execute the command.J
to join the lines again.
From :h :visual_example
:
Currently the ":" command works on whole lines only. When you select part of a line, doing something like ":!date" will replace the whole line. If you want only part of the line to be replaced you will have to make a mapping for it. In a future release ":" may work on partial lines.
See also :h visual-operators
for details.
From this comment: You can also try the vis or NrrwRgn plugin. Both should handle it just fine. (Thanks @christian-brabandt for the tip).