If your income goes up and your tax withholdings stay constant, you can see that trend early enough to begin saving up for a potentially higher tax bill. If your income declines, the inverse may also be true. The only way you'll know is if you're tracking the numbers (and with the help of your personal accountant's advice).
In the investing world, comparing year-to-date earnings from one year to the next is the best way to tell how well a company is doing over time. It's critical to understand how a company is growing and evolving, and the only way to do that is to track its performance on a trend basis.
Is revenue growing? Are margins improving or shrinking? How are external market events affecting the business relative to previous economic cycles? The answers to these questions are pertinent to understanding how a company has handled itself in the past, and they're indicative of what may happen in the future. Using prior years and trends is the only way to know.
It pays to compare apples to apples
The core of why year-to-date earnings are an important calculation is that it allows you to compare earnings or other metrics over the same period from year to year. Comparing how a company performed over the entire year last year with just six months this year doesn't make sense. That's apples and oranges.
Using year-to-date earnings allows you to compare the performance so far this year with the same period of time in years past. That's apples to apples, and for both your personal finances and your investing that's a valuable analysis tool.
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