confused.

I went to the doctors on Friday and they told me I was about 6 weeks and 3 days and sothay means tomorrow I'll be 7 weeks pregnant but my <a href="https://play.google.com/store/apps/details?id=com.glow.android">Glow app</a> says I'm 6weeks and 3 days today. is there a way I can Change how far along I am?