Because <<now>> is a javascript macro, it returns the formatted date as desired. There’s no need to use $wikify.
Tag values only need doubled-square brackets if they contain spaces. Thus, you can omit the newtags() macro, and just use tags="log" directly. If the tag value did contain spaces (e.g., “my log”), you could include the square brackets within the parameter value, like this: tags="[[my log]]"