1. Go to "My Biztory" - "Sales"
2. You can see under "Numbering", there's a "Sales Reference Number Format"
3. Over here you can customise your sales reference number.
{{YYYY}} = 4 digits year : {{2018}}
{{YY}} = 2 digits year : {{18}}
{{MM}} = Month : {{08}}
{{DD}} = Day : {{02}}
{{NUM4}} = 4 digits running number : {{0001}}
{{NUM3}} = 3 digits running number : {{001}}
{{NUM2}} = 2 digits running number : {{01}}
4. Example
5. Results show ( inv + 4 digits next running number + 4 digits year + month)