Long luteal phase

Laura • Married mother of 2 daughters (11 yrs & 6 weeks) and raising my 15 yr old niece.
If glow says my average luteal phase is 18 days why does it also say my period is due at 14 DPO? Waiting for 2 weeks is hard enough, but should I really be waiting for 18 days before I get concerned about actually being late?