> For the complete documentation index, see [llms.txt](https://docs.tryterra.co/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.tryterra.co/faq/help-topics/data-api-sdk/historical-data-and-backfill/historical-date-param-format.md).

# Why isn't the backfill / Sync Now button working?

When triggering a [historical](https://docs.tryterra.co/health-and-fitness-api/managing-user-health-data/requesting-historical-data) pull or backfill, the start and end date parameters must be a **plain date (`YYYY-MM-DD`)**, not a full ISO timestamp with time and milliseconds.

A full timestamp causes an `invalid isoformat` error and the request fails. **Strip the time portion before sending.**