Replacing Date with Datetime values

[ciscorucinski]

Description of proposed changes

The goal of this pull request is to replace the current dates with datetime + timezone offset.

Related issue(s)

Fixes #1

Potential Modifications

Convert Datetimes to UTC

• Currently, timezone offsets include +00:00, +01:00, +02:00, etc. But this modification would convert everything to be +00:00. NOTE: This will change the date of some variants such as XBB.1.5.14 which has a datetime of 2023-02-21T00:34:54+01:00 but its UTC date time is 2023-02-20T23:34:54+00:00.

Add Datetimes to variants prior to February 2021

• Currently, I only modified the variants that had existing dates associated with them. I don't know if there was a reason why you chose February 2021 as the cutoff, but I don't see a reason why this couldn't be extended. I can disable the timestamp checks and run the code during the night. Then I can reenable the timestamp checks and clean up any malformed variants

Use a different datetime format string

• Currently I use date.isoformat() which gives the result of 2023-02-21T00:34:54+01:00 for XBB.1.5.14. While I think this makes the most sense to follow ISO format, a more readable format string could also be used.

Testing

What steps should be taken to test the changes you've proposed?

• Verify that for future variants, my code produces an equivalent datetime string value. NOTE: the test fails because there have been 2 future variant updates (DV.3.1 and BF.7.14.7); however, they produce equivalent datetime string values!
• I could create a Google Sheet which verifies, for each variant, the current date part of the datetime string + timezone offset in my implementation matches the date in your implementation. I could also show which variants would have a date change.

Thank you for contributing to Nextstrain!

[corneliusroemer]

I'll have a look at some point, for now it's best to keep simple, I think