I tend to agree, the calendar is (generally) king. However, a lot of “time management” approaches say you must NOT put to do actions on the calendar (Getting Things Done is one that take this hard line). I also agree with them, up to a point. There are a lot of tasks that need to get done where you don’t really know how long they will take and they may depend on other factors, so scheduling is difficult. There is no point putting them on the calendar. On the other hand, if I need to get a significant task done, it gets a block of time on the calendar. How do I decide? Well, I use a Kanban board to manage tasks, if I have something that I know is going to need dedicated time, particularly if it has to be done by a specific time, it goes on the calendar. Using Kanban is also a way of mitigating the “too many to-dos” mentioned in other responses. You only move tasks to the “doing” column that you are working on or going to work on.
We also mustn’t forget that these systems are tools, they work for us, we don’t work for them. If I have a short amount of time, or limited resources, I might pick something off the Kanban backlog and clear it. The “system” says I shouldn’t, but it works for me, so I can and do.