Hi - First, have you seen your doctor? Most doctors want to talk to you if you have been TTC for more than 1 year without success. According to your post, you've been trying for almost 2 years. That warrants a trip to your doctor.
Once you have ruled out problems - have you tracked how long you go between periods each month (count 1st day to 1st day)? The average is 28 days, but women can vary widely from 22 to 36 days.
If your periods come very regularly exactly 28 days apart then yes, if the 1st day of your last period was Sept 14, then you would most likely ovulate between Sept 26-30.
But, if your periods are normally about 22 days apart, as you indicated happened last month, then your most likely ovulation date would have been Sept 22. Here is a website that can help you figure it out: http://www.askdramy.com/ovulationcalc.html
To use it properly, you have to track your periods for several months and know how long your average cycle is. If you are very irregular, then this method isn't going to work very well, and you might be better off getting the ovulation strips from the drug store that measure your hormone levels each day - it costs about $20 for a month's worth.
Good luck!!