Running this calculation results in an invalid value 0.006097 (correct value is closer to 0.120581416 according to google spreadsheets).
d = [{2038, 2, 4}, {2037, 11, 2}, {2037, 8, 2}, {2037, 5, 3}, {2037, 2, 4}, {2036, 11, 2}, {2036, 8, 2}, {2036, 5, 3}, {2036, 2, 5}, {2035, 11, 2}, {2035, 8, 2}, {2035, 5, 3}, {2035, 2, 4}, {2034, 11, 2}, {2034, 8, 2}, {2034, 5, 3}, {2034, 2, 4}, {2033, 11, 2}, {2033, 8, 2}, {2033, 5, 3}, {2033, 2, 4}, {2032, 11, 2}, {2032, 8, 2}, {2032, 5, 3}, {2032, 2, 5}, {2031, 11, 2}, {2031, 8, 2}, {2031, 5, 3}, {2031, 2, 4}, {2030, 11, 2}, {2030, 8, 2}, {2030, 5, 3}, {2030, 2, 4}, {2029, 11, 2}, {2029, 8, 2}, {2029, 5, 3}, {2029, 2, 4}, {2028, 11, 2}, {2028, 8, 2}, {2028, 5, 3}, {2028, 2, 5}, {2027, 11, 2}, {2027, 8, 2}, {2027, 5, 3}, {2027, 2, 4}, {2026, 11, 2}, {2026, 8, 2}, {2026, 5, 3}, {2026, 2, 4}, {2025, 11, 2}, {2025, 8, 2}, {2025, 5, 3}, {2025, 2, 4}, {2024, 11, 2}, {2024, 8, 2}, {2024, 5, 3}, {2024, 2, 5}, {2023, 11, 2}, {2023, 8, 2}, {2023, 5, 3}, {2023, 2, 4}, {2022, 11, 2}, {2022, 8, 2}, {2022, 5, 3}, {2022, 2, 4}, {2021, 11, 2}, {2021, 8, 2}, {2021, 5, 3}, {2021, 2, 4}, {2020, 11, 2}, {2020, 8, 2}, {2020, 5, 3}, {2020, 2, 5}, {2019, 11, 2}, {2019, 8, 2}, {2019, 5, 3}, {2019, 2, 4}, {2018, 11, 2}, {2018, 8, 2}, {2018, 5, 31}]
v = [-305, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, 188.0]
iex(91)> ExXirr.xirr(d, v)
{:ok, 0.006097}
Weirdly enough, if you just change the last -5 to -4.8 the calculation will return the expected value, like this.
d = [{2038, 2, 4}, {2037, 11, 2}, {2037, 8, 2}, {2037, 5, 3}, {2037, 2, 4}, {2036, 11, 2}, {2036, 8, 2}, {2036, 5, 3}, {2036, 2, 5}, {2035, 11, 2}, {2035, 8, 2}, {2035, 5, 3}, {2035, 2, 4}, {2034, 11, 2}, {2034, 8, 2}, {2034, 5, 3}, {2034, 2, 4}, {2033, 11, 2}, {2033, 8, 2}, {2033, 5, 3}, {2033, 2, 4}, {2032, 11, 2}, {2032, 8, 2}, {2032, 5, 3}, {2032, 2, 5}, {2031, 11, 2}, {2031, 8, 2}, {2031, 5, 3}, {2031, 2, 4}, {2030, 11, 2}, {2030, 8, 2}, {2030, 5, 3}, {2030, 2, 4}, {2029, 11, 2}, {2029, 8, 2}, {2029, 5, 3}, {2029, 2, 4}, {2028, 11, 2}, {2028, 8, 2}, {2028, 5, 3}, {2028, 2, 5}, {2027, 11, 2}, {2027, 8, 2}, {2027, 5, 3}, {2027, 2, 4}, {2026, 11, 2}, {2026, 8, 2}, {2026, 5, 3}, {2026, 2, 4}, {2025, 11, 2}, {2025, 8, 2}, {2025, 5, 3}, {2025, 2, 4}, {2024, 11, 2}, {2024, 8, 2}, {2024, 5, 3}, {2024, 2, 5}, {2023, 11, 2}, {2023, 8, 2}, {2023, 5, 3}, {2023, 2, 4}, {2022, 11, 2}, {2022, 8, 2}, {2022, 5, 3}, {2022, 2, 4}, {2021, 11, 2}, {2021, 8, 2}, {2021, 5, 3}, {2021, 2, 4}, {2020, 11, 2}, {2020, 8, 2}, {2020, 5, 3}, {2020, 2, 5}, {2019, 11, 2}, {2019, 8, 2}, {2019, 5, 3}, {2019, 2, 4}, {2018, 11, 2}, {2018, 8, 2}, {2018, 5, 31}]
v = [-305, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -5, -4.8, 188.0]
iex(102)> ExXirr.xirr(d, v)
{:ok, 0.120448}