Quite plainly, there is no straightforward correlation between the use of a present-tense form and the reference to present time.
(David Crystal, The English Language: A Guided Tour of the Language, 2nd ed. Penguin, 2002)

A simple definition of the simple present tense is that it expresses an action in present time.

But when it comes to English verbs, few things are that simple. As it happens, the present-tense form can refer to actions and events that occur not only in the present but also in the past, in the future, and outside of time altogether.

Consider these five additional uses of the deceptively "simple" present.

• Historical Present
  The historical present is used in the narration or reenactment of past events to create the effect of an immediate, eye-witness account. In practice, the present tense is rarely used by conventional historians, but a version of it--sometimes called the conversational historical present--often shows up in travelogues, sportscasting, and joke-telling: "A duck walks into a bar . . .."
• Literary Present
  The literary present is used in analyses of poems, plays, stories, and novels (regardless of when they were written) to indicate the apparent timelessness of literary works: "Chaucer emphasizes the knight's military skill . . ."; "Macbeth repeats his request for armor . . .."
• Gnomic Present
  The gnomic present is used to express a fact, belief, or general truth without reference to time: "The earth moves around the sun." The gnomic present is favored by the Bible ("Every good tree bears good fruit") and by contemporary social scientists ("Organizations seek to place their boundaries . . ."). "The advantage of the gnomic present," says economist and rhetorician Deirdre McCloskey, "is its claim to the authority of General Truth, which is another of its names in grammar."

• Habitual Present
  The habitual present is used to indicate an action that occurs regularly or repeatedly: "Every day the children leave for school in the dark." There's a timeless quality to the habitual present: the activity has occurred in the past and will continue to occur in the future.
• Future
  The simple present form can be used to indicate a future course of action: "Dave returns on Monday." (The present progressive--that is, a present form of "to be" plus a present participle--may also refer to future events: "The principal is retiring next year.")

To learn more about the peculiar habits of English verbs, see the following:

• Notes on Verbs
• Ten Types of Verbs
• Ten Quick Questions and Answers About Verbs and Verbals in English