I use an Excel spreadsheet as the central document for each customer. No stopwatch, I'm aware of how much I work and make an entry before stopping work or switching to a different customer. I used to be more granular on recording to the closest 15 minute increment, but that's tedious and allowing myself to be distracted isn't worth it, so I ensure the customer gets an even hour worth of work. I write down the date, hours, and short description of the work during that timeframe. If the customer has their own time management system, I record the day's entries into that at the end of the day. I record spreadsheet info into my QuickBooks invoices when it's time to bill. Then I create a new spreadsheet when starting a new invoicing period named with the date of the next invoice, e.g. CustomerName-20230327.xlsx.