Mailchimp datetime issue on 4.2

Just installed mailchimp free plugin on a 4.2 shop, but sync is not working.
log says:

http://developer.mailchimp.com/documentation/mailchimp/guides/error-glossary/ - Invalid Resource - Operation createOrUpdate-subscription-1-list-e71c069267 - timestamp_signup - This value is not a valid datetime.;

looked into it, found this issue which should have solved this problem I think:

https://github.com/nopSolutions/mailchimp-plugin-for-nopcommerce/commit/3f9b8b29da6a1301b796deb5897dd75c8dad227e

so what could be the cause?

i change this line of code        
            var member = new mailchimp.Member
            {
                EmailAddress = subscription.Email,
                TimestampSignup = subscription.CreatedOnUtc.ToString("u")
            };

to this

  var member = new mailchimp.Member
            {
                EmailAddress = subscription.Email,
                TimestampSignup = DateTime.UtcNow.ToString("yyyy-MM-dd hh:mm:ss")
            };

works on my end.