Change of status fro F2 to H4 in very short time

My wife is travelling first time to US on F2 visa on Sept 13th, 2019. I received H1b approval last month and my status will change from F1 to H1b on Oct 1st. I need to apply for my wife’s COS to H4 as soon as possible otherwise she will fall out of status staring Oct 1st. I am going to do it very next day she lands here but my question is - to save her from falling out of status, does the application (form I539) has to be submitted to USCIS before Oct 1st or the receipt notice has to be generated before that date?
Your inputs will matter a lot! Thank you in advance for your answer.

She will not be in out of status, she will be in period of authorized stay as she waits for her H4. You should put in the effective date of H4 as October 1st, so that it will be in sync with your F1 and H1B status. Also, discuss with your DSO on the same and do any changes required in SEVIS for the same with the DSO help. If USCIS receives the application before the effective date, it is fine…Read Period of Authorized Stay vs Out of status