ABAB XSLT Parse ISO 8601 Date with offset to Timestamp

We get timestamp data in XML as follows:

<creationtime>2017-11-27T03:01:53.593+01:00</creationtime>

How can we transform (XSLT) this to a valid UTC timestamp?

Note, if we chop off the first 14 chars SAP gets a timestamp but it's incorrect because of the missing +01:00