Sum Last 30 Days: How to Calculate It
The calculator for this page is switched off while it is checked. The formula is below, and the worked example in the article shows how to use it by hand.
This calculator works out the total sum of values that occurred in the last 30 days from a given date. You enter a list of dates with their amounts (or a single date and amount), and it returns the sum of those amounts whose dates fall within the 30-day window ending at your reference date. The result helps you track rolling monthly totals, such as sales, expenses, or workout minutes, without manually filtering a spreadsheet.
Formula
The sum of the last 30 days is calculated as:
Sum = Σ (amounti × indicatori)
where indicatori equals 1 if the datei is within the window, and 0 otherwise. The window is defined as:
Start date − days + 1 ≤ datei ≤ Start date
Variables:
- Start date – the reference date (YYYY-MM-DD), the last day of the window.
- Days – the length of the window (default 30).
- Amounts – the values to sum (e.g., dollars, units).
- Dates – the dates corresponding to each amount (YYYY-MM-DD).
Worked Example
Using the default values:
- Start date: 2025-01-01
- Days: 30
- Amounts: 100, 250, 75, 300
- Dates: 2025-01-15, 2025-01-20, 2024-12-25, 2025-02-01
Step 1: Determine the window start: 2025-01-01 minus 30 days plus 1 = 2024-12-03. So the window is 2024-12-03 through 2025-01-01 inclusive.
Step 2: Check each date:
- 2025-01-15 is after 2025-01-01 → outside, exclude.
- 2025-01-20 is after 2025-01-01 → outside, exclude.
- 2024-12-25 is between 2024-12-03 and 2025-01-01 → inside, include amount 75.
- 2025-02-01 is after 2025-01-01 → outside, exclude.
Step 3: Sum the included amounts: 75.
Result: The sum of the last 30 days is 75.00.
When to Use This Calculation
This tool is useful for any rolling 30-day total, which is common in business and personal tracking. Unlike a calendar month (e.g., January 1–31), a rolling 30-day window shifts every day, so it gives a more current picture. Examples:
- Tracking monthly sales or revenue on a daily basis.
- Monitoring expenses over a trailing month.
- Calculating workout minutes or calories over the past 30 days.
- Checking inventory usage or production output.
It is especially helpful when you want to compare today’s 30-day total with yesterday’s, to spot trends early.
How to Read the Result
The result is a single number: the total of all amounts whose dates fall within the 30-day window ending on the start date you entered. For example, if you enter today’s date as the start date, the result is the sum of the last 30 days from today. If you enter a past date, it gives the sum for the 30 days before that date.
Note that the window is inclusive of both endpoints. If you want exactly 30 days, the window starts 29 days before the start date (because the start date itself counts as day 1). The calculator uses start date − days + 1 to include the start date and the 29 days prior, giving a total of 30 days.
Common Mistakes and Misconceptions
- Mixing up “last 30 days� with “this month.� A calendar month has 28–31 days and starts on the 1st; a rolling 30-day window ends on your chosen date and includes the previous 29 days.
- Date format errors. Always use YYYY-MM-DD (e.g., 2025-03-15). If you use MM/DD/YYYY, the calculator may misinterpret the date, leading to wrong inclusion/exclusion.
- Forgetting that the start date is included. If you want the 30 days before a date but not including that date, subtract one day from the start date.
- Misaligned lists. The amounts and dates must be in the same order. The first amount corresponds to the first date, and so on. If they are not paired correctly, the sum will be wrong.
Limitations of the Formula
This calculation assumes that each amount is a single value associated with a single date. It does not handle recurring entries, partial-day timestamps, or time zones. If your data includes times (e.g., 2025-01-15 14:30), you should convert to just the date to avoid excluding a time that falls after midnight. Also, the formula treats all days equally; it does not account for business days or holidays. For financial reporting, you may need to define “last 30 days� differently (e.g., 30 business days), which would require a more complex calculation.
Another limitation: the calculator only sums amounts that have dates. If you have a date with no amount, it is ignored. If you have an amount with no date, it is also ignored. Ensure every amount has a corresponding date.
Frequently Asked Questions
What does “last 30 days� mean exactly?
It means the 30-day period ending on a specific reference date, counting backwards. For example, if the reference date is March 15, the last 30 days are February 14 through March 15 inclusive. This is a rolling window, not a calendar month.
Can I use this for a different number of days, like 7 or 90?
Yes, you can change the “Days� input to any positive integer. The formula adjusts automatically. For a 7-day window, use 7; for 90 days, use 90. The same logic applies.
What if my dates are in the future?
Future dates are outside the window (since they are after the start date) and will be excluded. If you want to include future dates, you would need to set the start date to a later date.
How do I handle multiple entries on the same date?
Simply list each entry as a separate amount and date. The calculator sums all amounts that fall within the window, regardless of whether they share a date. For example, two amounts on the same date will both be included if that date is inside the window.
Does the calculator account for leap years?
Yes, the date arithmetic handles leap years automatically. For instance, February 29 is a valid date in leap years, and the window calculation will include it correctly.