serial_number - Required. The date from which to extract the day of month (1-31). Can be a date value, cell reference, or text date that Excel recognizes.
| A | B | |
|---|---|---|
| 1 | Date | Day Number |
| 2 | 1/15/2025 | |
| 3 | 2/28/2025 | |
| 4 | Formula: | =DAY(A2) 15 |
Get day number from date
=DAY("1/15/2025") → 15
Flag 15th of any month
=IF(DAY(1/15/2025)=15) → "Due"
Calculate days until month-end
Returns days left in current month
Categorize by half-month
=IF(DAY(1/20/2025)<=15) → "Period 2"
Identify all invoices due on the 15th of every month for automated payment reminders and cash flow planning. The DAY function in Excel enables finance teams to flag specific billing dates regardless of month or year, ensuring timely payments and preventing late fees. This Excel DAY formula approach is critical for accounts payable automation, vendor payment tracking, subscription billing systems, and credit card payment schedules where consistent monthly dates drive business operations and maintain vendor relationships.
| A | B | C | |
|---|---|---|---|
| 1 | Invoice | Due Date | Reminder |
| 2 | INV-001 | 1/15/2025 | |
| 3 | INV-002 | 2/28/2025 | |
| 4 | INV-003 | 3/15/2025 | |
| 5 | Formula: | =IF(DAY(B2)=15, "Send Reminder", "") Send Reminder |
Categorize transactions into first-half (days 1-15) and second-half (days 16-31) periods for payroll processing and semi-monthly financial reporting. The Excel DAY function automatically splits monthly data into two billing periods, essential for companies that pay employees twice per month, process vendor invoices on bi-weekly schedules, or generate mid-month and month-end reports. This DAY formula in Excel simplifies period-based aggregation for accounting systems, time tracking applications, and commission calculations that operate on semi-monthly cycles.
| A | B | C | |
|---|---|---|---|
| 1 | Transaction | Date | Period |
| 2 | Sale #1 | 1/8/2025 | |
| 3 | Sale #2 | 1/20/2025 | |
| 4 | Formula: | =IF(DAY(B2)<=15, "Period 1", "Period 2") Period 1 |
❌ The Problem:
✅ Solution:
=DAY("1/15/2025")Always provide a complete date with day, month, and year. The DAY function in Excel requires a valid date serial number to extract the day component. Use "1/15/2025" or "January 15, 2025" format that Excel recognizes as a complete date value.
❌ The Problem:
✅ Solution:
=WEEKDAY(TODAY())Use WEEKDAY function for day of week (1-7 where 1=Sunday). The Excel DAY formula extracts the day number within the month (1st through 31st), while WEEKDAY identifies which day of the week it is. Choose DAY for billing dates and payment schedules, WEEKDAY for work scheduling and weekend detection.
❌ The Problem:
✅ Solution:
=IF(ISDATE(A1), DAY(A1), "Invalid")Validate that the input is a date first, then extract the day. The DAY function in Excel automatically respects month boundaries - February never returns 31, April never returns 31, etc. Focus validation on whether the cell contains a valid date using ISDATE or ISNUMBER combined with date tests.
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