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In Webster's Dictionary，"One and a half years have passed since then." and "A year and a half has passed since then." are described.
I am confused between the "one and a half years have" and "a year and a half has".
Why is different "one and a half years" and "a year and a half" in singular or plural?
I will be happy if you could explain it in detail. Thank you in advance.

Any numerator greater than one (such as 'one-and-a-half') precedes a plural noun and governs a plural verb. In the case of 'a year and a half' the noun 'year' is preceded by a singular numerator (one), and the additional 'half' is added subsequently. In such cases despite the mathematical total being the same, grammatically the noun is singular and therefore governs a singular verb.