I see nothing wrong with variations on the following:
"I am working on _____ right now. I will be able to get this task complete for you by _____. Is that acceptable?"
"I need to finish this task by ____ and then I can look at this and will be able to tell you when I can get this done. Is that ok?"
If boss-boss says no, it needs to be done NOW or similar. Then you're expectations change and you should let your boss know your priorities have shifted.
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