CODEWITHSUNDEEP
datetimejuliadates.jl
Bex T.
Bex T.

Reputation: 1786

How does `Dates.value` function work in Julia?

I learned that Dates.value(Date) returns an integer representation of that date.

using Dates

dt = Date(2020, 01, 01)

>>> println(Dates.value(dt))
737425

How does it go from 2020-01-01 to 737425?

Upvotes: 2

Views: 274

Answers (1)

Bogumił Kamiński
Bogumił Kamiński

Reputation: 69939

The value is a field in this structure:

julia> dump(dt)
Date
  instant: Dates.UTInstant{Day}
    periods: Day
      value: Int64 737425

The Dates.value function extracts this field.

For the Dates.Date object Date(1, 1, 1) is associated with value equal to 1, and then consecutive days have value increasing by 1.

However, Dates.value is defined for other objects. E.g.:

julia> Dates.value(Time(1))
3600000000000

julia> Dates.value(Second(10))
10

julia> Dates.value(Minute(20))
20

The fact that Dates.value does not perform this computation is important as it ensures that the operation is fast.

If you want to see how this is calculated for Date, check the source code of Dates.totaldays function which calculates this using Rata Die system:

# Convert y,m,d to # of Rata Die days
# Works by shifting the beginning of the year to March 1,
# so a leap day is the very last day of the year
const SHIFTEDMONTHDAYS = (306, 337, 0, 31, 61, 92, 122, 153, 184, 214, 245, 275)
function totaldays(y, m, d)
    # If we're in Jan/Feb, shift the given year back one
    z = m < 3 ? y - 1 : y
    mdays = SHIFTEDMONTHDAYS[m]
    # days + month_days + year_days
    return d + mdays + 365z + fld(z, 4) - fld(z, 100) + fld(z, 400) - 306
end

Upvotes: 4

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