Make the interest a separate transaction & date it in 2018.

Resolution for '2018 transaction issue: 'We have an account billed 2017 – when I add the interest in 2018 it shows as a 2017 transaction. How do I get the interest added to be recorded as a 2018 transaction?”' available: Yes.

The 2018 transaction issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the 2018 transaction issue yourself by looking for a resolution described in the article. If it is a complex 2018 transaction issue or you are unable to solve the issue, you may contact us by clicking here or by using other 2018 transaction support options.

For more information about resolving issues related to 2018 transaction in Quickbooks/QBO or to get support for the issue 'We have an account billed 2017 – when I add the interest in 2018 it shows as a 2017 transaction. How do I get the interest added to be recorded as a 2018 transaction?”', click the link below: