1)
iTF.frames[unitID].castBar:SetMinMaxValues(startTime/1000, endTime/1000)
iTF.frames[unitID].castBar:SetScript('OnUpdate', function()
    iTF.frames[unitID].castBar:SetValue(GetTime())
end)

2)
iTF.frames[unitID].castBar:SetMinMaxValues(startTime/1000-GetTime(), endTime/1000-GetTime())
iTF.frames[unitID].castBar:SetScript('OnUpdate', function()
    iTF.frames[unitID].castBar:SetValue(GetTime()-startTime/1000)
end)