The 22nd would only be the required date if the SEC allowed "Business" days, which they don't.
The 5 days ended on Sunday and since the SEC only allows reports to be sent to them on work days, they allow the report to be filed on Monday, the 21st.
Due date: 15th
Followed by the 5 days: 16th, 17th, 18th, 19th and the 20th (Sunday)
My posted comments are only my considered opinion based on the reality as I see it. Your reality may be different.