Teach this to your manager: At the beginning of a task, uncertainty is highest. Under no circumstances should you give an estimate in ‘man-hours’. Even days is too precise. The first estimate should be in months or years (of course depending on the size of the project). Then, as your insight into the project grows, you refine that to months, then weeks, later days. A vague estimate with a lower and a higher bound is way more useful to your manager than a ridiculously ‘precise’ but highly speculative number.
This lesson was brought to you by either “Code Complete 2” or “Rapid Development” by Steve McConnel, and by my former manager who wanted projects estimated in minutes.