SPARQL dateTime subtraction with a particular format

Question

Here we see that when I try to subtract 2 datetime with angular brackets it doesnt seem to work but if the angular is removed it works fine.

what I am currently doing is generating triplets via a python library and it creates the triplets with angular brackets (serialization in turtle) and query runs after it, ofcourse we have tried a quick fix to fix the file before the query but is there a better way to do it?

PREFIX : <http://example.com/abc/abc#>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
select ?diff where { 
    
    BIND("2020-09-21T12:47:50Z"^^<xsd:dateTime>  as ?diff1).
    BIND("2020-09-20T12:47:50Z"^^<xsd:dateTime>  as ?diff2).
    BIND(?diff2 - ?diff1 as ?diff).
}

Update

work around for this

BIND(xsd:dateTime(STR(?diff2)) - xsd:dateTime(STR(?diff1)) as ?diff).