@Scenn *******
Your extension of stay/ entry stamp is what gives you the right to be inside Thailand, not when the visa expires. As of right now, you're on amnesty; and you can stay no later than September 26, OR get an extension of stay. You can get either:
a. An extension of stay based on marriage, which requires the 400K THB in savings in your bank account for 2 months / 40K a month in income (assuming you're male and married to a Thai female). It'll let you stay for a year, with the effective date starting September 27
or
b. An extension of stay based on visiting Thai family. This does NOT require the 400K, but you can stay for 60 days, effective September 27. This also DOES NOT void your existing visa.