The windows_task resource utilizes the schtasks command in cmd. If you issue the following command in cmd: schtasks /Create /SC DAILY /TN test /TR filename_%date:~10,4%%date:~4,2%%date:~7,2%_%time:~0,2%%time:~3,2%%time:~6,2%.txt
the schtasks.exe will evaulate the date and time values.
So the question is not on chef side I think, but on how to escape it for schtasks.exe.
BTW I do not understand why are you giving a text file as a command