Sounds like you have customizing invoice down pat. At this time, adding a calendar is not an option. You could add a custom field and manually enter the information based on the format you want, which you may have already tried. I appreciate your willingness to provide feedback and product suggestions. From within your QuickBooks account:
Choose Help from the top menu
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Your voice and suggestions are so important to product enhancements. Our developers read each and every product verbatim and all feedback, so thanks for taking the time to submit.
If you have any other questions, please feel free to drop a comment here. I’d be happy to help.