I don't know if the answer above was applicable to earlier versions of splunk. But as for 7.0.1 I did a diff on the entire etc directory before and after doing the change in the UI. And here is what I found:
diff -r etc/etc/apps/user-prefs/local/user-prefs.conf etc2/etc/apps/user-prefs/local/user-prefs.conf
2,3c2,3
< default_earliest_time = 0
< default_latest_time =
Basically modify/create /opt/splunk/etc/apps/user-prefs/local/user-prefs.conf
default_earliest_time = 0
default_latest_time =
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