The biggest thing the date data type brings in my opinion is you can index that attribute on a node which if you're writing queries to fetch nodes between a date range, it's a lot simpler to accomplish on an indexed attribute. Otherwise with time trees you're using the unique nature of graph traversals to return nodes in a traversal path.
What gets complicated with time trees is maintaining the tree. How much of the tree do you initialize? Do you build the tree organically as you have data on the time range? What granularity do you build the tree, do you build it all the way down to the minute and second or do you only build down to the day?
In the end you're model is heavily dependent on how you query your graph. If you use time trees or a datetime data type, it's up to how you need to query your graph.