That's correct - you can directly add/subtract a certain number to axis bounds as they are numeric. The values correspond to the OLE Automation date equivalent, retrieved by the DateTime.ToOADate
method. In OLE Automation date equivalent a value of 1 corresponds to a time span of 1 day, so adding/ subtracting 365/366 would do the job, but you can also use this code myDate.AddYears(1).ToOADate.
You can find an example of manipulating X axis range with date time values here
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