🗓️ Date Addition & Subtraction Tool

🎯 What is the Add/Subtract Days Calculator?

The **Add/Subtract Days Calculator** is a precision utility that determines a future or past date based on a starting point and a specified number of days. Its most valuable feature is the ability to calculate dates using **Business Days** (Monday through Friday), which is critical for planning deadlines, payment due dates, and project timelines.

💡 Why You Need This Tool and Its Purpose

Standard calendar arithmetic often fails when professional deadlines need to exclude weekends. This tool solves common date calculation challenges:

1. **Project Planning:** Determine a reliable project completion date by adding 60 business days.
2. **Financial Deadlines:** Calculate the final due date for invoices or legal notices that specify a period in business days.
3. **General Scheduling:** Quickly find a date 90 days or 180 days in the future, regardless of how many weekends are involved.

⚙️ How This Calculator Works: Business Day Logic

The tool uses standard JavaScript date objects for **Calendar Day** calculations, which simply add or subtract the total number of 24-hour periods. The complexity lies in the **Business Day** calculation.

1. Calendar Day Calculation:

For a starting date ($\text{D}_{\text{start}}$) and a number of days ($\text{N}$), the formula is straightforward: $$ \text{D}_{\text{final}} = \text{D}_{\text{start}} \pm \text{N days} $$

2. Business Day Calculation (Excluding Weekends):

When adding or subtracting business days, the algorithm must iteratively check the day of the week for each step:

• If the current day is a weekday (Monday-Friday), the calculation proceeds to the next day, and the count ($\text{N}$) decreases by 1.
• If the current day is a weekend (Saturday or Sunday), the calculation skips the day(s) until it reaches the next Monday, but the count ($\text{N}$) **does not** decrease.