@labanina I think the way they phrased it is super awkward...so it's easy to overlook the possibility that they two ladies are speaking more in between them because MC asks "When was your last period?" and there's no cohesive answer from the other lady other than "It starts today" (It's quite possible that the writer honestly had no idea when the fertility window was when they wrote this too)
If I was the mangaka's editor, I'd probably re-write the MC's dialogue bubble "It's closely related to the moon cycle and those times are calculated off of it" and replace it with "It's closely related to the moon cycle so we can calculate your fertility time off of it. How you can figure it out is...
blah blah blah" so that way the reader would realize that MC is explaining to the other lady about time frames.
I'd also probably edit the answer "It starts today" to "...Then that time would start today." So the 'Then that time' would imply she is accounting for what MC had just explained to her.
Again, not criticizing author or translation. Fertility times is uncommon knowledge and easily misunderstood and I've seen MANY people make this mistake before (and regret it) so I just wanted to clarify it to prevent misunderstandings about such an important time XD