Some ways:

  • Enter future-dated transactions in your journal, commented out (with ; or comment)

  • Enter future transactions uncommented; use a query to exclude them from reports when needed (-e tomorrow or date:-tomorrow. hledger-ui hides them by default.)

  • Enter future transactions in a separate forecast.journal, which you can include when needed (eg, add -f forecast.journal).

  • Enter periodic transaction rules describing future transactions (recurring or non-recurring), and activate the forecast with --auto.

  • Budgeting and forecasting (2018) > Forecasting - reusing a budget's periodic transactions to generate a forecast.