Time
The current frame number, time, delta time, and wall-clock values.
What it does
Time is a clock wired into other nodes. It has no inputs and reports where the graph is in playback and in the real world, every frame. Frame, Time and Delta track the timeline being played or rendered: the current frame number, the elapsed time, and how much time passed since the last frame. Hour, Minute, Second, Day of Week and Day of Year read the computer's wall clock instead.
Nothing here is visible on its own. Every output is a number for an Expression, a parameter wire or a Gradient, not a layer.
When to use it
An animation depending on frame number or elapsed time, from Frame or Time
Per-frame motion that reacts to how fast frames are arriving, such as frame-rate-independent motion, from Delta
Something changing with the time of day or day of week: a clock face, a schedule-driven look, a business-hours switch, from the Clock outputs
A value that ticks over the course of a year, from Day of Year
Gotchas
Day of Week counts Monday as 0 and Sunday as 6, not the Sunday-first convention some calendars use. Offset it if your logic assumes Sunday is day 0.
Day of Year runs 1 through 366 inclusive, not a zero-based index. Subtract 1 to use it as an array index.
Delta is the gap since the previous frame, so it is the output to multiply speeds by for motion that holds pace regardless of playback frame rate.
Worked example
Add a Time node.
Wire its Time output into a rotation or offset parameter on another node to animate it over playback.
Wire Hour and Minute into an Expression to build a live clock display, or into a colour Gradient to shift a look across the day.