I always provide my managers with confidence interval estimated times
50% 10 days (assumes *nothing* goes wrong, no interruptions, and high code reuse)
90% 15 days (assumes no catastrophic issues, no interruptions > 2 hours and only 5 of them, moderate code reuse)
100% 55 days (the wheels fell off, severe schedule impacts of interruptions non stop, no code reuse)
My boss laughed at my 100% estimate until it actually happened.
A lead dev (who could be counted on for sound advice delivered in a belittling way) was struck down by lung cancer and ceased to exist. Another lead dev who was even brighter, and much nicer to work with was poached by a competitor, both within days of each other. The code was cutting all new territory in the system, so maybe 15% reuse? *and* some panicky manager started having $deity damned _daily_ meetings about it.
We almost missed my 100% date, made it by about 16 hours.