Mailchimp have made an adjustment to their API that currently impacts Synchronizations and Import processes on Chimpegration Classic. This adjustment will result in the error message: System Error: Dates must be in a valid ISO 8601 time format.
We have contacted Mailchimp and are currently working on a solution with them.
In the meantime, we would recommend using Chimpegration instead and using the Import and Export features found there. Chimpegration does not offer a Synchronisation feature, instead, we would recommend considering setting up scheduled or real-time process to help automate your import and export processes.