This implementation features:

1. A clean, modern design with subtle shadows and rounded corners
2. Responsive layout that works well on both desktop and mobile
3. Color-coded calendar highlighting:
   • Period days in pink
   • Fertile window in light green
   • Ovulation day in green
4. Clear legend explaining the color coding
5. Input validation and easy-to-use form
6. Smooth animations and hover effects
7. Professional typography and spacing
8. Shadow effects for depth and visual hierarchy

To use the calculator:

1. Select the first day of your last period
2. Enter how many days your period typically lasts
3. Enter your average cycle length
4. Click “Calculate” to see your predicted fertile window, ovulation day, and next period dates in the calendar view

The calendar automatically generates the relevant months and highlights the important dates using the color-coding system. The layout adjusts dynamically based on the calculated dates.

The calendar for February would show 12-16 as period days? Wait, no. The last period is Feb 12-16, but the user inputs that as the last period. The calculator is for predicting the next period and ovulation. So the next period is March 12-16. The ovulation is in February. So the February calendar should show the fertile window (21-26) and ovulation (26). The March calendar shows the period days (12-16).

But according to the code, the periodStart and periodEnd are nextPeriodStart and nextPeriodEnd, which are in March. So in the code, the period days would be highlighted in March.

The last period (Feb 12-16) is input by the user but not highlighted in the calendar. The calculator is for future dates, so the code is correct in highlighting the next period and fertile window.

So the code should work.

Potential issues:

– Date handling across month boundaries (e.g., if ovulation is in a different month than the fertile window start).

– Daylight saving time changes affecting date calculations. Using setDate and UTC could help, but JS Date handles local time.

– The calendar generation for months with varying starting days.

– Styling to ensure responsiveness on mobile devices.

But overall, this approach should work. Now, putting it all together in the final answer.

Here’s a beautifully designed Period Calculator with an intuitive interface and calendar visualization: