What timezone is the “timestamp” value in the Databricks Usage log ?

Is it UTC?

timestamp

2020-12-01T00:59:59.000Z

Need to match this to AWS Cost Explorer timezone for simplicity.

It's UTC.

Please see timestamp under Audit Log Schema https://docs.databricks.com/administration-guide/account-settings/audit-logs.html

https://docs.databricks.com/administration-guide/account-settings/billable-usage-delivery.html we doesn't state it explicitly under Step 4: Access the log files for analysis.

However in https://docs.oracle.com/javase/8/docs/api/java/text/SimpleDateFormat.html you have:

For parsing, 

"Z"

 is parsed as the UTC time zone designator. General time zones are not accepted.