Converting between units of time
Converts a duration between seconds, minutes, hours, days, weeks, months, and years. Months and years use average lengths (30.44 days and 365.25 days respectively, accounting for leap years) since calendar months and years vary in exact length - for most planning and estimation purposes, these averages are what you want.
Why a single fixed conversion factor is used for months despite real months varying
Calendar months range from 28 to 31 days depending on which month and whether it's a leap year, which means there's no way to convert "1 month" to days with perfect precision without knowing which specific month is meant - using the average (365.25 ÷ 12 = 30.44 days) gives a consistent, leap-year-aware approximation that's accurate for planning purposes, even though it won't exactly match any single real calendar month.
Why years use 365.25 days instead of a flat 365
The extra quarter-day accounts for the leap year added roughly every four years - averaging it in this way means a duration converted to or from years stays accurate over longer spans, rather than slowly accumulating error if every year were naively treated as exactly 365 days.
Why this matters for project planning and deadline estimation
Converting between different time units comes up constantly when planning - estimating how many weeks a project phase will take in days, or how many hours a multi-week task represents - having every unit converted simultaneously, rather than computing one conversion at a time, makes it easy to sanity-check a duration across several different framings at once.