Publishing sensor data to MongoDB - data objects for time series

I have set up a Webhook to publish data to my MongoDB database but the published_at information is being sent as a String. This seems to create problems when doing date queries in MongoDB. Does anyone have any suggestions on how to either send it as a date object or convert it to a date object on the receiving side?
I have also set up a small node js server (express) on my Raspberry Pi so I think that could be another option but I am quite new to Express. Thanks

How does your insert statement look?
Have you tried Date(yourDateString)?