Data type not detect correctly for latest_event_time in Mongodb for tracker store

I am using Mongo DB for tracker store. And i created some charts using this MongoDB charts. When i create a chart using the field latest_event_time it showing 'Invalid Date` in the chart. When changing the data type of that field (latest_event_time) from double to date it works fine. By default rasa store the field as double. Is it bug ? or any other way to resolve this?