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You can go to Setting>User Interface>Time Ranges>Add New (or edit it, if you have one in place) and place the earliest time at
@w0 and latest time to
@w4and you can make this a public global time range
Copy the stanzas you want to change from $SPLUNKHOME/etc/system/default/times.conf to $SPLUNKHOME/etc/system/local/times.conf. Then change "@w1" to "@w0" and "@w5" to "@w4" in the appropriate places. You'll probably have to restart Splunk for the changes to take effect.