This formula is clever, but it is also brittle. The user who posted it is doing something many of us have done: trusting a video tutorial without understanding what the formula actually assumes. The core issue is that `WORKDAY` and `NETWORKDAYS` by default treat Saturday and Sunday as the only weekend days. That is fine for a standard Monday-to-Friday calendar. The problem is that the outer `WORKDAY` function is being fed a sequence of numbers that counts only working days, but it then maps those numbers back onto a timeline that includes weekends. When the first of the month falls on a weekend, the inner `WORKDAY(EOMONTH(A3,-1),1)` correctly skips to the next Monday. The sequence of working days then lines up. But when the first of the month is a weekday, the formula sometimes shifts the entire output by one day, causing weekend dates to appear in the calendar grid.
What this means for anyone building a dynamic calendar is that you cannot copy a formula from a video and assume it works for every month. The formula shown here is a clever combination of date math, but it lacks a critical safeguard: it does not validate that the resulting dates actually fall within the target month. A more robust approach would involve using `SEQUENCE` to generate a grid of all days in the month, then filtering those days with a `WEEKDAY` check to exclude weekends. Alternatively, you can use `WORKDAY.INTL` to explicitly define weekend days, which gives you control over the behavior. The formula as written is a shortcut, and shortcuts fail when edge cases appear.
Our take is straightforward: stop treating formulas as black boxes. The user here is learning, and that is exactly the right attitude. But the lesson is not about fixing this one formula. It is about understanding that spreadsheet tools like `WORKDAY` and `NETWORKDAYS` are context-sensitive. They assume a specific calendar structure, and when your data does not match that assumption, they will produce inconsistent results. The solution is not to hunt for a perfect formula online. It is to break the problem down: generate a list of dates, then filter them explicitly. A `SEQUENCE` of all days in the month, combined with a `FILTER` that removes weekends, will work for every month without surprises.
The concrete point here is about control. Dynamic calendars are useful, but only when you understand the conditions under which your formulas work. If you cannot explain why a formula behaves differently in January versus February, you have not mastered it yet. Mastery comes from testing edge cases, not from following videos.