Learn Hardcore German: Die genaue Uhrzeit steht in meinem Kalender.

Because it’s a weak adjective ending. After the definite article die in the nominative singular feminine, adjectives take the ending -e, giving die genaue Uhrzeit.

What is the difference between Uhrzeit and Zeit?

Zeit refers to time in general (the concept or duration), whereas Uhrzeit specifically means the clock time (e.g. 14:30). So you need Uhrzeit when you talk about a precise hour and minute.

What does the verb stehen mean in this context?

Here stehen doesn’t mean “to stand” physically but “to be written” or “to appear.” So steht in meinem Kalender means “is written in my calendar.”

Why is the preposition in followed by the dative case here?

In can govern either accusative (for movement into something) or dative (for location). Since nothing is moving—you’re stating where something is—you use dative: in meinem Kalender.

Why is it meinem Kalender and not meinen Kalender?

Kalender is masculine. In the dative singular, masculine nouns take dem, and the possessive pronoun mein matches that with the ending -em, resulting in meinem Kalender.

Could you start the sentence with In meinem Kalender instead?

Yes: German allows fronting a prepositional phrase. You’d say In meinem Kalender steht die genaue Uhrzeit. The finite verb (steht) still stays in second position.

Why not use liegen instead of stehen to express “is in my calendar”?

When referring to written entries—on paper, in a list, in a calendar—German conventionally uses stehen, not liegen. So you say steht im Kalender, not liegt im Kalender.