Waking from a four year coma David is told his daughter just died. He joins a counselling group, but it's three months before his daughter dies. He wants to change his daughters future, the counsellor argues it isn't possible.

I respectfully disagree with MC's feedback. Where we are that the original logline is too wordy and needs to punch us in as few words as possible, I don't find the counselor plug to be essential to the logline because it's too "on the nose". Of course, a counselor will say it's impossible - readers are smart enough to think that.

How about this:

After the death of his daughter, a father awakes from a four year coma and learns he has three months to prevent his daughter?s demise.?

Your take on it reads:
After the death of his daughter the father awakes from a four year coma. And the father awakes from a four year coma before his daughters death.
The counsellor's role is vitally important to the whole story.
With regard to the one sentence theory, I agree.
But to get the logline right, the full stops should be included initially.
If you look at many of the loglines here a full stop has simply been replaced with a comma. In one of them the word temptation still has a higher case letter after a comma.

One thing that may merit a second sentence (or a minor lengthening of the logline) is some additional conflict. Audiences are pretty savvy to time travel stories at this point. We know the story will have one of two outcomes: the past is static and unchangeable, or there are multiple outcomes and the past can be altered.

I'd suggest adding another bit really explaining what conflict makes this story unique.