When to test?!

I'm having a hard time deciding when to test, my original fertile days were 9/4-9/10, had some red spotting for two days after then nothing. Have been having mild cramping since then and period was due 9/23, now I'm late but when I update my period log my fertile days change...so how do I know how many days past ovulation if it keeps changing? Can we get a how many days late button lol. Please help!!