How does rust chrono Duration get number of years or months? - date

Question

I'm quite new to Rust and chrono library.
I checked the https://docs.rs/chrono/0.4.19/chrono/struct.Duration.html#method.num_weeks, but there's no num_years() or num_months() API in chrono::Duration.
Is there any work around solution for this ?

Answer 1

chrono::Duration provides date and time duration in "ISO 8601 time duration with nanosecond precision", which implies it is representing the duration internally as a number of nanoseconds, and then providing convenience methods to convert into other duration units such as days, weeks, milliseconds, etc.
This is a little at odds with the actual ISO 8601 duration standard, which is a standard of representations and formats. The standard represents durations by the format P[n]Y[n]M[n]DT[n]H[n]M[n]S - which might give you what you want. But this is not what chronos::Duration was designed to provide.
The problem is that in order to represent a duration of months or years, more information than the number of nanoseconds is needed. Durations define the amount of intervening time in a time interval: the time between two points. The start or end time is important, because months or years are not standard durations. There are months of 28, 29 30, 31 days, and years of 365 and 366 days.
If you were to write your own algorithm to format durations in terms of years, months, days, hours, minutes, seconds, etc.. you would have to know the start date. In addition, time zone is important, because daylight savings needs to be taken into account. You would also have to make decisions about how to represent parts of months or years. For example the month of January has 31 days, and February 28, say. What would it mean to represent a duration of 1.75 months from January 1? Would that mean 31 days for January then 0.75 * 28 days in February?
Or you could represent the duration from a start date in a cascading unit format: e.g., 5 years, 4 months and 3 days, 2 hours, 12 minutes and 3 seconds from 1 Jan 1970 12:00Z. Just like the ISO 8601 standard.
So, its not an easy solution, and it all depends what your requirements are. I can understand why the developers of chronos:Duration left off providing num_months() and num_years()!

Answer 2

The crate chronoutil provides RelativeDuration which is "extending Chrono’s Duration to add months and years".
Example from docs.rs:
let one_day = RelativeDuration::days(1);
let one_month = RelativeDuration::months(1);
let delta = one_month + one_day;
let start = NaiveDate::from_ymd(2020, 1, 30);
assert_eq!(start + delta, NaiveDate::from_ymd(2020, 3, 1));