First i would thank CKPT for this amazing article that post in his blog .

Instead of reviewing whole alert log file for any Particular information, We can search using that key word from X$DBGALERTEXT Table.

Grep The word “Starting Up” of Alert log file from Instance.

col ORIGINATING_TIMESTAMP for a40
col MESSAGE_TEXT for a80
set linesize 500
SELECT
originating_timestamp,
message_text
FROM
sys.x$dbgalertext
WHERE
message_text LIKE '%Starting up%';

Grep The word “Instance shutdown complete” of Alert log file from Instance.

SELECT
originating_timestamp,
message_text
FROM
sys.x$dbgalertext
WHERE
message_text LIKE '%Instance shutdown complete%';

again thank you CKPT for this article since its useful for all DBA’s , you can check CKPT Blog From the above link

Thank you
Osama mustafa