Hello Duyen,
That setting is mainly for maintenance which might be relevant if the logs are older then X.
The setting that controls the index time frame is found in $INDEXDIR/log_indexer_custom_setting.conf.
You need to remove these lines:
:time_restriction_for_fetch_all (1458723037)
:time_restriction_for_fetch_all_disp ("23/Mar/2016 10:50:37")
and replace it with this one:
:num_days_restriction_for_fetch_all_integrated (30) // change 30 with the number of days needed.
You'll need to restart the indexer by running in expert: evstop;evstart
Hope that helps,
Yonatan