Many freelancers - but not limited to - are using time recording apps to document worktimes
I suggest to support the app time recording since it is the most flexible app (in viewpoint of output data like excel sheets): https://play.google.com/store/apps/details?id=com.dynamicg.timerecording&hl=en
The app can export data via mail, dropbox, Google Drive and so on.
Ideally the workflow would be as follows:
-> In InvocePlane select the exported excel sheet to import the data
(Perfect solution: InvoicePlane can query via IMAP/POP a specific mail account to pull the reports send by the app.)
-> InvocePlane parse the sheet and creates a list of times worked for each customer recorded in the sheet.
-> Not known customers can be created on the fly
-> User select the times he wants to put on the invoice
-> InvoicePlane creates for each customer and selected times an invoice (template)