title: How to use Timestamp_to_scn and Scn_to_timestamp in Oracle?
body: <p>I have this as a result of the query:</p>
  
  <pre><code>select cast(to_date(a.start_time,'mm/dd/yyyy hh:mi:ss pm') as timestamp) date_of_call,
  ora_rowscn from calling_table a where rownum &lt;= 10;
  
         DATE_OF_CALLING          ORA_ROWSCN
  
  26-JUL-13 12.29.28.000000000 PM 8347567733892
  26-JUL-13 12.29.35.000000000 PM 8347567733892
  26-JUL-13 12.29.35.000000000 PM 8347567733892
  26-JUL-13 12.29.38.000000000 PM 8347567733892
  26-JUL-13 12.29.44.000000000 PM 8347567733892
  26-JUL-13 12.29.47.000000000 PM 8347567733892
  26-JUL-13 12.29.48.000000000 PM 8347567733892
  26-JUL-13 12.29.48.000000000 PM 8347567733892
  26-JUL-13 12.29.48.000000000 PM 8347567733892
  26-JUL-13 12.29.56.000000000 PM 8347567733892
  </code></pre>
  
  <p>But when I try to convert this timestamp into scn using the function timestamp_to_scn, 
  I am getting the following error:</p>
  
  <p>ORA-08180: no snapshot found based on specified time
  ORA-06512: at "SYS.TIMESTAMP_TO_SCN", line 1
  08180. 00000 -  "no snapshot found based on specified time"
  *Cause:    Could not match the time to an SCN from the mapping table.
  *Action:   try using a larger time.</p>
  
  <p>And when I am using scn_to_timestamp on ora_rowscn to convert that column into a timestamp, I am getting the following error:</p>
  
  <p>ORA-08181: specified number is not a valid system change number
  ORA-06512: at "SYS.SCN_TO_TIMESTAMP", line 1
  08181. 00000 -  "specified number is not a valid system change number"
  *Cause:    supplied scn was beyond the bounds of a valid scn.
  *Action:   use a valid scn.</p>
  
  <p>What is it that I am doing wrong?</p>