The uptime is great if your workstations are reliably checking in. 
Convert(Decimal(7,2),tblAssets.Uptime / 60 / 60 / 24) As [Days up]
The issue is when you want to know WHEN was the last reported reboot.
Laptops may not have reported in for a couple weeks, yet still show an uptime of less than one day.
You can calculate the 
last boot time with the tblAssets.Uptime and blAssets.Lastseen
tblAssets.Lastseen - Convert(Decimal,tblAssets.Uptime / 60 / 60 / 24)   As [Boot Time]
I tend to be fairly verbose in my reports. 
Select Top 1000000 tblAssets.AssetID,
  tblAssets.AssetName,
  tsysAssetTypes.AssetTypeIcon10 As icon,
  Convert(Decimal(7,2),tblAssets.Uptime / 60 / 60 / 24) As [Days up],
  tblAssets.Lastseen - Convert(Decimal,tblAssets.Uptime / 60 / 60 / 24)   As [Boot Time],
  tblAssetCustom.Location,
  tblAssets.Lastseen,
  Left(tblADComputers.OU, CharIndex(',', tblADComputers.OU) - 1) As OU,
  '<img src="thumbnail.aspx?user=' + tblADusers.Username + '&domain=' +
  tblADusers.Userdomain + '&size=12" class="rimage"/>' As Picture,
  tblADusers.Displayname,
  tblADusers.Username,
  tblADusers.Userdomain,
  tblAssetCustom.Model,
  tblOperatingsystem.Version As [OS Version],
  tblOperatingsystem.Caption As [OS Name],
  tsysIPLocations.IPLocation,
  tblAssets.Description As [LS Description],
  tblADComputers.Description As [AD Description]
From tblAssets
  Left Outer Join tblComputersystem On tblAssets.AssetID =
    tblComputersystem.AssetID
  Left Outer Join tblADusers On tblAssets.Username = tblADusers.Username
  Left Outer Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
  Left Outer Join tsysAssetTypes On tsysAssetTypes.AssetType =
    tblAssets.Assettype
  Left Outer Join tblADComputers On tblAssets.AssetID = tblADComputers.AssetID
  Left Outer Join tblOperatingsystem On tblAssets.AssetID =
    tblOperatingsystem.AssetID
  Left Outer Join tsysIPLocations On tsysIPLocations.LocationID =
    tblAssets.LocationID
Where tblComputersystem.Domainrole < 2 And tblAssetCustom.State = 1 And
  tblAssets.Lastseen - Convert(Decimal,tblAssets.Uptime / 60 / 60 / 24) < Getdate() - 14
Order By tblAssets.Lastseen - Convert(Decimal,tblAssets.Uptime / 60 / 60 / 24) Asc, tblAssets.Uptime Desc,
  tblAssets.AssetName