HPPS migration: started_at timezone conversion inflates days-to-complete calculations

[donnapep]

Summary

The HPPS data migration converts the start comment meta value using convert_to_utc(), but the source value is already in local time (matching comment_date). The conversion adds the GMT offset instead of subtracting it, pushing started_at into the future relative to the UTC-based completed_at. This inflates DATEDIFF calculations for Days to Completion in Reports.

Root cause

In Student_Progress_Migration::prepare_lesson_progress_rows():

• completed_at ← comment_date_gmt (already UTC)
• started_at ← convert_to_utc(meta['start']) — but start is stored in local time (same timezone as comment_date), so the conversion shifts it in the wrong direction

Example (site with gmt_offset = 6)

	Comments (local time)	Tables (after migration)
start	2022-05-23 22:47:51	2022-05-24 03:47:51 (+5h shift)
complete	2022-05-23 22:47:51	2022-05-23 16:47:51 (UTC)
DATEDIFF + 1	1 day	2 days (crosses midnight)

The start meta value 2022-05-23 22:47:51 is local time. convert_to_utc() treats it as local and adds +6h → 2022-05-24 04:47:51. Meanwhile completed_at is comment_date_gmt = 2022-05-23 16:47:51 (UTC). The resulting started_at is 11 hours after completed_at.

Impact

• Days to Completion values on the Reports > Lessons page are inflated when using tables-based storage
• The magnitude depends on the site's GMT offset — larger offsets cause more dates to cross midnight boundaries
• Comments-based storage is unaffected (both comment_date and start meta are in local time)

Possible fix

The start meta value should be treated the same way as comment_date — either:

1. Don't convert it (store as-is, since comment_date is stored as created_at without conversion), or
2. Subtract the offset instead of adding it (convert local → UTC correctly)

Option 1 would also require removing CONVERT_TZ from the tables-based queries that compare started_at and completed_at, since they'd be in different timezones. Option 2 is cleaner but requires a data migration for existing sites.

Steps to reproduce

1. Set up a site with gmt_offset > 0
2. Create a course with a lesson and quiz
3. Have students complete the lesson
4. Enable HPPS and run the migration
5. Compare Reports > Lessons "Days to Completion" between comments-based and tables-based storage