In reading up on this, messing abound in the tables is not supported by Microsoft. USE AT YOUR OWN RISK!
I attempted the very same thing. When you change a subscription on the front end, the ReportServer.dbo.Schedule.StartDate column changes with each edit. I tried just altering this value, but adding a value here made no difference. There was no email and when I went back to my subscription in the SSRS webpage, the original datetime value was there. So that datetime must be stored elsewhere.
I did a SQL Trace and found these Sproc's were being called. I copied out the lines that looked promising. By changing the time value @active_start_time=160400 in the last one and running the entire batch, I could get an emailed report at the desired time. But I am not content with this; because when I go back and view My Subscriptions webpage, the original datetime is still listed.
exec msdb.dbo.sp_delete_job @job_name=N'D478A346-ED05-422D-A73E-023080AD56DD'
exec msdb.dbo.sp_add_job @job_name=N'D478A346-ED05-422D-A73E-023080AD56DD',@description=N'This job is owned by a report server process. Modifying this job could result in database incompatibilities. Use Report Manager or Management Studio to update this job.',@category_id=101
exec msdb.dbo.sp_add_jobserver @job_name=N'D478A346-ED05-422D-A73E-023080AD56DD',@server_name =N'(LOCAL)'
exec msdb.dbo.sp_add_jobstep @job_name=N'D478A346-ED05-422D-A73E-023080AD56DD',@step_name=N'D478A346-ED05-422D-A73E-023080AD56DD_step_1',@command=N'exec [ReportServer$instanceDB].dbo.AddEvent @EventType=''TimedSubscription'', @EventData=''db7d7b08-3eee-4bb8-b354-b58c653b8ab6'''
exec msdb.dbo.sp_add_jobschedule @job_name=N'D478A346-ED05-422D-A73E-023080AD56DD',@name=N'Schedule_1',@freq_type=1,@active_start_date=20180405,@active_start_time=160400,@freq_subday_type=1