This is an ISO 8601 standard for time duration - see “Duration” at https://en.wikipedia.org/wiki/ISO_8601
Specifically for P2DT4H
‘P’ - this is a time period. The following string may be D for days, Y for years, M for Months (though BigFix may only support a subset, not sure we allow Years or Months)
2D - 2 Days
T - the following is a Time duration
4H - the duration is an additional 4 Hours
So this duration is 2 days + 4 Hours