Is there a number to call that doesnt cost $3 per minute? I emailed the complaint department, but I leave in two weeks and need to get this issue resolved quick.
No, there's no number - and the number that does cost $3 per minute (Worldbridge) isn't going to be of any help to you anyway as they can only help with priority processing and how to apply for a visa.
I believe all you can do is email the complaints address and hope for the best.
But are you sure there's actually been an error made on your visa? If I remember correctly, you put a travel date of September 23rd, but your visa was made valid from August 16th.
However, did you actually make it explicitly clear on your application form that you wished for the visa to be made valid from September 23rd and that you were specifically requesting a post-dated visa?
They are supposed to make your visa valid from the travel date you put on the form, but they don't always stick to this - if you don't make it very clear in your application that you want it to be post-dated, they will sometimes make the visa valid from the date they issue it... and in this case, I'm not sure that it can be considered an error or that they will correct it.
It isn't generally expected that someone will spend a full 6 months in the UK on a visit or visit for marriage visa... they usually expect that you will travel to the UK, spend a few weeks there so you can marry and then leave again well before the 6 months is up, so they aren't going to make an effort to ensure that you have a full 6 months to spend there.
Also, as it is advised that you should not book any flight until you have your visa, I would guess that they just assume that you will book your travel as soon as you get your visa, and so if it's issued earlier than expected, it means you can either still travel when originally planned date, or you can book your flights to leave earlier if you want to.