Go into Webmin -> Servers -> BIND DNS Server -> Module Config, choose Zone File Options from the dropdown, and from there you can set "Serial number style" to "Date based (YYYYMMDDnn)".
After one does this and applies the changes to BIND, how does one apply this date based serial number style to all existing BIND DNS Server configurations?

 

A quick way to update after changing the serial format so all dns records contain the newly selected format is to go to [webmin] -> [BIND DNS Server] then select all the dns zones and click [Update Records In Selected] - I then chose to adjust the TTL to the same value that it already was, which updated all the record files.