With -GetTime(): (works)
iTF.frames[unitID].castBar:SetMinMaxValues(starttime-GetTime(), endtime-GetTime())
  iTF.frames[unitID].castBar:SetScript('OnUpdate', function()
  iTF.frames[unitID].castBar:SetValue(GetTime()-starttime)
end)

Without -GetTime():
iTF.frames[unitID].castBar:SetMinMaxValues(starttime, endtime)
  iTF.frames[unitID].castBar:SetScript('OnUpdate', function()
  iTF.frames[unitID].castBar:SetValue(GetTime())
end)