Strange case - How will USCIS decide the H4 end date?

[uhhuduga]

Hello, 

My wife quit her job last year and applied for a COS from H1B to H4 based on my H1B (primary applicant) petition in July 2017. Back then my H1B was only valid till Jan 2018 and that was what was submitted to USCIS . As of today's date  , we are way past Jan 2018 . If my wife's H4 gets approved today, how will USCIS determine the last date ? Typically the last date on the H4 matches with the primary applicant's last date. Since Jan 2018 has already passed , how will USCIS determine the last date? Will it retrograde the last date on the H4 to Jan 2018 effectively making my wife out of status for 4 months?