Following info here:
http://sdkdocs.roku.com/display/sdkdoc/ifDateTimeI came up with this (untested):
FUNCTION getDateDiff(myISODate1, myISODate2) AS INTEGER ' dates are ISO 8601 strings
date1 = CreateObject("roDateTime")
date2 = CreateObject("roDateTime")
myDate1 = date1.FromISO8601String(myISODate1)
myDate2 = date2.FromISO8601String(myISODate2)
myDate1Sec = myDate1.AsSeconds()
myDate2Sec = myDate2.AsSeconds()
diffSec = myDate2Sec - myDate1Sec
diffDay = diffSec\86400 ' Using \ instead of / will return an integer, ignoring remainder/fraction/decimal
RETURN diffDay
END FUNCTION
But I've never used roDateTime, so I might be doing it wrong. And I don't know how to get your dates into ISO 8601 strings in the first place.