Excel (at least Excel 2016, probably most older versions also) can add times to a datetime cell by adding a correctly formatted string, for example A1 contains Datetime 1-Jan-17 12:00 PM A2 contains Formula =A1+"1:30" In Excel, this results in the content of A2 being 1-Jan-17 1:30 PM (as a Date). In XSSF, this results in an error (#VALUE).
this seems to work in the latest POI code - just added an XSSF test - r1894175