Learn Swedish: Bokningen är inte i kalendern. - The booking is not in the calendar.

You just need to add a future time phrase at the end! For example, Bokningen är inte i kalendern nästa vecka (The booking isn't in the calendar next week). In Swedish, using a present tense verb like är alongside a future time word is the most common and natural way to talk about the future.

Why is "the calendar" written as kalendern instead of kalenderen?

The base word is en kalender. When an en word already ends in an unstressed -er, Swedish drops the extra e before adding the definite ending -en. This becomes kalendern, which flows much better when speaking.

How do I know whether to use -en or -et to make "the booking" (bokningen)?

The base word is en bokning. A very helpful trick to remember is that almost all Swedish nouns ending in -ing (which are usually formed from verbs, like att boka) are en words. Therefore, you will always use the -en ending to make them definite.

Why does inte come after the verb är here?

This is a standard main clause, so the verb must be in the second position (the V2 rule). The subject (Bokningen) is in the first position, the verb (är) is in the second, and sentence adverbs like inte usually plug in directly after the main verb.