You picked a task. Make room for it.
You know what you want to do. Then three other things ask for attention. Start a session from the task or preset you chose and keep a record of the time you actually gave it.
Start from the work you chose
Pick a task or a preset, then start its focus session. Keep the timer with that piece of work. When you finish, complete the session; when you step away, pause it. The recorded duration gives you a concrete account of time spent without asking you to remember it later.
Pause without losing the thread
Say you start on a report, then need to take a call. Pause the session, return to the report afterward, and complete the session when that piece of work is finished. The interruption does not erase the time already spent.
Give the next task a clean start
In experiments on task switching, unfinished work could keep some attention on the earlier task. A session will not prevent interruptions, but pausing it gives you a clear place to return.
Let the record be descriptive
A timer cannot tell you whether a day was good. It can show the time you spent, so you can compare your plans with reality without guessing.