How far along according to Glow App

Lisa-Marie

Im in need of some explications. From what I understand , to calculate how far along one is in pregnancy, you calculate from the first day of you last period. My last period was dec 30th which on my calculations means Im 5 weeks. The glow app says 5 weeks and one day but the glow nurture app says 4 weeks and 3 days ?! Does anyone know what is right ?