Manage Alert in Projeqtor

Indicate date B as validated end date.
You can then have an alert : create an indicator for the "Respect of validated end date".
You can choose to send a warning (for instance 1 day before target date) and and alert where target date is reached (alert at 0 day) or passed (alert at -1 day or -1 hour).

In addition, when planning, if planned end date is over date B (defined as validated end date), planned end date will appear in red background, and task bar in Gantt will also be red.
This will be done even if current date is lower than date B, and this is important to plan late dates. When current date B is over date C, it's too late.