Currently a calculated column in a list report can be formatted as a date and can be included in a custom roadmap in the "Shown on bars" and "Group bars by" section which is helpful.
However, the calculated column is not available to select as a date field (e.g. the bars start/end date), even though the calculated column is formatted as a date. It would be helpful to be able to have the bars start/end dates be driven by a calculated column formatted as a date as well.