In a video posted to the prime minister’s official Twitter feed after parliament was prorogued, Boris Johnson asserted that only four days of parliamentary sitting time would be lost.
Without any prorogation, parliament would have been in recess for, at most, 13 days over September and October for conference recess (from 16 September to 7 October, assuming a four-day week). This assumes parliament returning on the Tuesday after conference season as it did last year.
Parliament was prorogued in the early hours of 10 September with the intention it would return on 14 October for the Queen’s speech.
The plan for prorogation would have meant Parliament would have been in recess for a minimum of 19 days (the 13 stated above and additionally the 10 to 12 September and 8 to 10 October).
So, in total, parliament would have lost at least six sitting days if the prorogation had taken place. Moreover, no substantive parliamentary business could have taken place during the Queen's speech on 14 October. In practical terms, at least seven days of parliamentary sitting time would have been lost.
The Prime Minister also received advice from his Director of Legislative affairs that seven sitting days would be lost (she did not count the Queen’s Speech day but assumed Parliament would return on the Monday after conferences, rather than the Tuesday).
This was a lie. Once parliament was prorogued on 10 September, it became inevitable that Parliament would lose a minimum of six sitting days. It’s worth saying here that Boris Johnson’s false claim, published on his official Twitter profile, was made at the height of the controversy over his decision to prorogue parliament. He was in a position to know the truth but instead gave voters false information. There was therefore no excuse whatsoever for Boris Johnson conveying false information to voters on his official Twitter feed.