The initial cycle length input is there so that Glow’s algorithm can learn your baseline. However, this information is only used initially, and once you start to input actual cycle dates, Glow will use that data to predict your future periods.
If you’re wondering how your initial cycle length stacks up with your averages in Glow, go to Home > Period/Edit Period and look at the ‘List’ view on your calendar. There, you’ll see the cycle lengths that you’ve previously reflected in your start and end date inputs. If these look incorrect, reach out to us at firstname.lastname@example.org and we can help you get this figured out! If they’re spot on and matching with your calendar dates and your memory, you may just have a different cycle length than you thought :)