Does Marketo allow importing values from an old datetime field to a new SFDC custom date field.
The error you're showing isn't from importing a converted value but from trying to copy a value as-is.
If you export, truncate the field to only the date portion, and actually reimport (from a file) you won't have this problem. Marketo won't say a Date-like string value is an "invalid token" for import into a Date field.
Bear in mind that conversion from DateTime to Date is always lossy, that's why it is not usually allowed when the formats of both sides are known. If you truncate "2018-07-01T23:59:00" to just "2018-07-01" then convert again to DateTime, you get "2018-07-01T00:00:00" -- almost 24 hours off from the original So it is understandable that this is not something to silently allow.