I'm not sure, but I think it takes some time before users can attach bounties to unanswered questions. Is this time calculated from the moment the question was initially asked, or the time of the most recent edit of the OP? And how long is that time period?
What is meta?
- Any Stack Exchange user can ask a question
- The community provides support, votes on ideas, and reports bugs
- Your voice helps shape the way Stack Exchange operates
yes the time is calculated from the time when question is posted first time.
From FAQ -
The time is calculated from the initial asking of the question.
From the "How does the bounty system work?" question.