How can my wife set start date of menstrual cycle if different then perdicted dates?

I switched my wife over to Garmin connect with a garmin venu 4 from fit-bit and other apps so her info could be all in one app. Having difficulty setting start date of her menstrual cycle which was two day ahead of prediction. Is this a software issue that just needs updating or are we missing the magic bottom somewhere. Thank you otherwise for such great tech and app. Hope someone in development or internal it gets this.