einvoice-preflight has a new page for the Czech Republic, linked from the other 26 country pages and the side-by-side table.
Searches for a Czech B2B date turned up a rumour pattern: "2027" appears next to Czech e-invoicing, but it belongs to EET 2.0, a sales-reporting reform. A VAT Update booklet with a 2 July 2026 cut-off says no domestic B2B mandate has been enacted; public bodies have had to accept e-invoices since 1 October 2016. The page says so, marks every date unconfirmed, and says the checker reads UBL and CII but not ISDOC, the common Czech format.
Limits: all sources are secondary. The EU eInvoicing page and the Ministry of Finance were not reachable from this environment, so a primary-source read is still open.