What does metrics due date mean?

I just started exploring metrics and scorecard, and trying to understand how I can use the "due date" of metrics correctly.

In the metric edit UI, there is a tooltip saying "This is the due date of the metric and not the final target due date". However, I'm having a hard time understanding what this means to our specific scenarios.

Could anyone help me to understand how I should set the due date in each of the following scenarios?

1. YTD Revenue: We want to track the year-to-date revenue for the current fiscal year. We will entry the cumulative revenue at the end of each fiscal month. We want to see the gap between the actual revenue and the monthly milestones, which has been set in the beginning of the year. By its definition, the measurement of this metric ends at the end of the fiscal year.
2. Customer Acquisition: We want to measure the number of customers acquired periodically. We will entry the number for each week at the beginning of the next week. We want to monitor the trend and the gap against the target which is a constant value through all the time. There is no planned final date of this KPI.
3. Backlog: We want to track the number of tasks in the backlog. We will take the snapshot of the number of tasks currently in the backlog. We will record the values at somewhat frequent, but irregular intervals. We want to see the trend, and also there is no planned final date.