A simple definition of time is it's that thing that a clock measures. But which clock and how fast is it traveling, i.e. relativity theory says that the faster a clock is traveling the slower it runs compared to a stationery observer. Time is manufactured by consciousness, it's something we create.
If one thinks of time as a sphere with you in the middle in the "now" then every point on the surface represents a distinct time. Everything is happening at once and you can reach any point in time with the proper tools. There is no beginning of time and no end. Every point in time and space is connected to every other point. Everything is happening at once.
A persons perception of time is affected by their situation and mental state. Stress can make time stretch out. Isolation can do the same as some noticed during the pandemic. Certain drugs can affect our sense of time.
The past is over and cannot be changed although each persons memory of an event may be different. That does not change the event itself. The future is not predetermined but based on the probabilities of a number of possible events. The choices one makes will determine which events actually are experienced.