Shares of Advanced Micro Devices (AMD) are down in after-hours trading after the chipmaker reported Q4 earnings results.

There's no specific rule governing the timing of earnings reports, but most companies choose to release their results within a few hours before the stock market opens or after it closes.

The P/E ratio helps investors determine the market value of a stock compared with the company’s earnings. It shows what the market is willing to pay for a stock based on its past or future earnings.