in tenses,doesn't it sound odd-

"I will have done it before that happens."

'I will have done it before that happens' is an example of future perfect tense. The rule of making future perfect tense is - [will have+past participle]. It expresses the idea that something will occur before another action in the future.

Here in the example, we have used the future perfect tense to express the idea that the intended work would have been completed by the subject(I) before the start of the second task.

Hope it helps.