“What causes the menstrual cycle to change dates?”

Normal cycles are between 26-30 days. Otherwise, see a gynecologist for investigation.

Well, although, typically, women's menstrual cycles are every 28 days, this is not true for lots of women. Although, there can be reasons for women having irregular periods such as physical stress, becoming ill, emotional stress, obesity, and polycystic ovarian syndrome, many other women just have irregular menses. There is a large variety of reasons. If you are concerned, you should talk to your OB/GYN, however, it is not a reason to become concerned. 
I hope this helps. 

Warmest Regards,  

Joseph A. Adashek, MD FACOG

Changes in menstrual cycle dates could be multifactorial. However, it is worth of note menstrual cycle is not a single date, but is a 21-35 day cycle and occurrence of menses in this time window is considered normal. That said salient causes could be stress, diet changes and lifestyle changes, nutritional changes, hormonal changes such as thyroid conditions may be implicated. At best, if this is a concern, visit your doc for a thorough work-up.