How to get SCN from database. Timestamp against SCN

In many cases we need to know SCN of database. Also we may need to know the previous SCN of the database suppose before 10 minutes ago. I will try to write we can can get these two and how is this correlated.

A)Determine current SCN of The database.
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You can query from V$DATABASE or using GET_SYSTEM_CHANGE_NUMBER procedure of DBMS_FLASHBACK package.

1)SQL> SELECT DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER() FROM DUAL;
DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER()
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1053658

2)SQL> SELECT CURRENT_SCN FROM V$DATABASE;
CURRENT_SCN
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1053668

B)Determine current timestamp value.
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SQL> SELECT SYSTIMESTAMP FROM DUAL;
SYSTIMESTAMP
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14-MAY-08 11.00.08.374107 PM -04:00

C)Convert SCN to Timestamp.
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SQL> SELECT SCN_TO_TIMESTAMP(1053639) FROM DUAL;
SCN_TO_TIMESTAMP(1053639)
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14-MAY-08 10.52.15.000000000 PM

D)Convert Timestamp to SCN
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SQL> SELECT TIMESTAMP_TO_SCN('14-MAY-08 11.00.08.374107 PM') FROM DUAL;
TIMESTAMP_TO_SCN('14-MAY-0811.00.08.374107PM')
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1054516