I am getting a
TimeSpan ts = XmlConvert.ToTimeSpan("P72H");
System.FormatException was unhandled by user code
Message=The string 'P72H' is not a valid TimeSpan value.
You need to add the Time separator to your string. Try this:
TimeSpan ts = XmlConvert.ToTimeSpan("PT72H");
See the duration specification - http://www.w3.org/TR/xmlschema-2/#duration
126.96.36.199 Lexical representation
The lexical representation for duration is the [ISO 8601] extended format PnYn MnDTnH nMnS, where nY represents the number of years, nM the number of months, nD the number of days, 'T' is the date/time separator, nH the number of hours, nM the number of minutes and nS the number of seconds. The number of seconds can include decimal digits to arbitrary precision.