This question was asked before, but the answer given was how to do it with Apex, and I am not using Apex:

How to get the current timestamp using SOQL query?

How do I do the equivalent of getServerTimestamp() in Apex, as either a SOQL query or through the REST API?

I need the server timestamp to record when I last do a sync with data that resides on Salesforce and my local database. I will then query for records that have a system modified timestamp later than the last sync. If I use my local timestamp, I risk and difference in clocks resulting in some modifications being missed.

Thanks for your help.