Infopath calculate time difference between two inputs

Need to get the number of hours between two timeinputs in infopath. This will always be on the same day.
60 * (substring(laterTime, 12, 2) - substring(earlierTime, 12, 2)) + (substring(laterTime, 15, 2) - substring(earlierTime, 15, 2)) + (substring(laterTime, 18, 2) - substring(earlierTime, 18, 2)) div 60
When pasting this to a field in infopath, we first need to remap latertime and earliertime to relevant fields in our own form. When trying to verify the formula it throws an error stating a missing ,
Upon closer examination of the xpath values we notice than an extra . is in place where the , should be .

When the dots are are replaced with commas the code works as it should.
References:
http://www.infopathdev.com/forums/p/20732/71795.aspx