Days to years (Julian)
One day is 0.002737850787 years (Julian), and this shows the working rather than just the answer. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
The factor
One day is 0.002737850787 years (Julian). That is the whole of the conversion: multiply by it, and nothing else happens.
And it is exact
Not rounded, not measured, not a convention that might be revised. The factor above is the definition, so an answer from it is as exact as the number you typed.
The day is exactly 86,400 seconds. A real solar day wanders by a millisecond or two.
The Julian year is exactly 365.25 days, which is what astronomers use. A calendar year is not a fixed length.
The number in your head
Most people use 0.0027 for this, which is the factor to two significant figures. It is low by 1.3825% — 13.825 years (Julian) too few in every thousand, and 138.25 in every ten thousand.
Going the other way
The reverse is not this sum backwards in your head. It is a different factor, 365.25, and dividing by one is not the same as multiplying by a rounded version of the other.
Rounded, the way back is 365, which is low by 0.0684%. So the two roundings do not cancel: go out and back with both and you do not land where you started.
How many of those decimals mean anything
The box above will give you ten decimal places because it can. Whether they mean anything is decided by your measurement, not by the arithmetic.
If you measured to the nearest day, your figure is already uncertain by half of one — so the answer is uncertain by about 0.001369 years (Julian), and every digit past that is decoration. Quoting 0.00338004 when you measured with a tape is claiming a precision the tape never had.
Where these two sit
Days and years (Julian) both measure time, which is why they can be converted at all. A conversion between families — a length into a weight — is not a conversion, it is a question with a missing quantity in it.
The unit converter pairs days with hours rather than with years (Julian), because that is the conversion people ask for most. This page exists because a good many ask for this one instead.
Other conversions from days
minutes · hours · weeks · seconds · milliseconds