Getting Wrong values when updating the column based on Date Difference value is zero

Getting Wrong values when updating the column based on Date Difference value is zero.

I am getting * value in the check number column.

  UPDATE #temp2
  SET CheckNumber = UC.CheckNumber
  FROM #temp2 T2
 inner join UserDb.dbo.ucy uc on  UC.ClaimID = t2.claimID
where DATEDIFF(second, UC.[date], t2.[date]) = 0