Thats not so easy to achive, because there is no option for this in the "Configure-->Report Options..." menu.
You could change the "Add Start Time XML" Step in the "AddReportHeader" Sequence in the ...\National Instruments\TestStand 3.1\Components\NI\Models\TestSTandModels\reportgen_xml.seq Sequence-File.
Another way would be to adjust the Stylesheet (...\National Instruments\TestStand 3.1\Components\NI\Models\TestSTandModels\StyleSheets\)
For both you need knowlwedge about XML...