This is possible to do in Splynx. As you know we have a Preview of all invoices that will be generated.
For example you are generating the invoices 1st of July. So the Preview is run 1st of July, then you have some days to confirm and then it creates the invoices.
Splynx allows you to run the preview in advance, for example 24th of June and generate invoices that are for July.
To enable this feature you need to activate Allow saving preview in future.
Config -> Finance -> Automation
There you also define Set date to finance elements - it says what date will appear in Invoice. In our case if we generate 24.06 for July :
Real Date of issuing = 24.06
Billing Date = 1.07