Migrate away from arbitrary reference dates

[sophiamersmann]

Unblocks #2007

Summary

What: Migrate away from specifying dates as the number of days since an arbitrary reference date that is either chosen by an author or defaulting to 2020-01-21
Why: To make internal date representation more sane (and also to prepare for future work supporting more time intervals)
How: Substantial database migrations, plus some work in the Grapher codebase

Current state

• Time points are represented as integers and either...
  • interpreted as years
  • interpreted as the number of days since a reference date (if the metadata field display.yearIsDay is true)
    • the reference date is either taken from display.zeroDay
    • or the default reference date is used (EPOCH_DATE): 2020-01-21

Concerns

• Specifying dates with reference to an arbitrary date in Jan 2020 is weird/unexpected
• Arbitrary reference dates add unnecessary complexity

Actionable plan

Step 1: Change the default reference date

• Write a database migration: For all variables without an explicit zeroDay, set the zeroDay to the the current default date, 2020-01-21
• Change the EPOCH_DATE in Grapher to 1970-01-01

Step 2: Clean up database and make sure all variables use the new default reference date (1970-01-01)

• Write a database migration: For all variables with daily data, update the time column to use the new default reference date (1970-01-01)
• Remove display.zeroDay

(optional) Step 3: Store years and dates in different columns

• Add a new column data_values.date
• For all variables with daily data, move time points from data_values.year to data_values.date

[marcelgerber]

Makes sense!

PS: I changed display.zeroDate to display.zeroDay in the above text, which is what the field is actually called :)

[danyx23]

We talked abou this - it's probably a lot of work for relatively little benefit. It is probably better to concentrate on building a new, better solution for dealing with time as part of #2007 and then deprecate yearIsDay or migrate away from it if that is worthwhile.