As you noted and per KB Article 555936; The Active Directory stores date/time values as the number of 100-nanosecond intervals that have elapsed since the 0 hour on January 1, 1601 till the date/time that is being stored. The time is always stored in Greenwich Mean Time (GMT) in the Active Directory.
Using that information and the reference here, the formula would be
You should be able to put formula into an eval statement for Splunk.
I asked about the Add-On for Active Directory because there is an easier method to retrieve this data by changing your ldapsearch to return the computed password expiry time in standard format. Here is a sample of what that would look like.