use units::si

@name("Week")
@url("https://en.wikipedia.org/wiki/Week")
@aliases(weeks)
unit week: Time = 7 days

# The mean tropical year changes over time (half a second per century). It's current
# value can be approximated using
#
#   365.2421896698 - 6.15359e-6 T - 7.29e-10 T^2 + 2.64e-10 T^3
#
# where T is in Julian centuries, measured from noon January 1st, 2000.
# (https://en.wikipedia.org/wiki/Tropical_year#Mean_tropical_year_current_value)
#
# Values of the mean tropical year for the recent past and near future:
#
#   Year    Length (days)
#   ---------------------
#   2020    365.242 189 7
#   2025    365.242 188 1
#   2050    365.242 186 6
#
# For now, we use the 2025 value as a hardcoded constant. Those numbers
# are mainly shown to illustrate that it is not sensible to define this
# number more precise.
#
@name("Tropical year")
@url("https://en.wikipedia.org/wiki/Tropical_year")
@metric_prefixes
@aliases(years, yr: short, tropical_year, tropical_years)
unit year: Time = 365.242_188_1 days

@name("Month")
@url("https://en.wikipedia.org/wiki/Month")
@aliases(months)
unit month: Time = year / 12

@name("Gregorian year")
@url("https://en.wikipedia.org/wiki/Gregorian_year")
@metric_prefixes
@aliases(gregorian_years)
unit gregorian_year: Time = 365.2425 days

@name("Julian year")
@url("https://en.wikipedia.org/wiki/Julian_year_(astronomy)")
@aliases(julian_years)
unit julian_year: Time = 365.25 days

@name("Decade")
@url("https://en.wikipedia.org/wiki/Decade")
@aliases(decades)
unit decade: Time = 10 years

@name("Century")
@url("https://en.wikipedia.org/wiki/Century")
@aliases(centuries)
unit century: Time = 100 years

@name("Millennium")
@url("https://en.wikipedia.org/wiki/Millennium")
@aliases(millennia)
unit millennium: Time = 1000 years