Let me explain using example forecast of profit over date.
Follow the below steps:
1. Create a calculative field to fetch dates already existing in dataset + values till Forecast date.
2. Create a forecast calculated field to create a forecast of data till the required date using the above created field.
3. Now drop the date field created in step 1 in column shelf and field created in step2 in row shelf.
Thanks to TableauHQ for his explanation.