Milestones seem to be falling at a breakneck pace over the past couple of years. On Monday, Feb. 13, the broad-based S&P 500 (SNPINDEX:^GSPC) closed at an all-time record high, and in the process, the aggregate value of S&P 500 companies crossed the $20 trillion threshold for the first time ever. For Wall Street, the milestone represents affirmation that the bull-market rally is here to stay.

However, it took quite some time for the S&P 500 to double based on market cap from $10 trillion to $20 trillion – 19 years -- given the dot-com bubble in 2000-2001, as well as the Great Recession. Here are just some of the things that've changed over those 19 years.

Stock traders cheering in front of computer screens.

Image source: Getty Images.

  • Apple (NASDAQ:AAPL) stock: In 1998, the technology giant was still nearly a decade from introducing the world to the iPhone, and it controlled about 0.06% of the S&P 500's weighting. Today, at a better than $700 billion market cap, Apple and its army of loyal customers is responsible for about 3.5% of the S&P 500's weighting.
  • Gallon of gas: A gallon of gas would set you back $1.07 by the end of January 1998, whereas today the average gallon of gas costs $2.31.
  • Bars of gold and silver.

    Image source: Getty Images.

    Price per ounce of gold: At the end of January 1998, you could pick up an ounce of gold for $302.02. Today, that same ounce of gold will cost you about $1,228.30.
  • Price per ounce of silver: Toward the end of January 1998, an ounce of silver could be purchased for just $6.22. As of today, that same silver ounce is worth $17.94.
  • Movie ticket price: Heading to the movies in 1998 would have cost an average of $4.69 on an inflation-adjusted basis, compared with an estimated $8.65 per ticket today.
  • Median home price: If you were purchasing a home in January 1998, you were dealing with a median home price of $148,000, according to the Census Bureau. As of the third quarter of 2016, the median home prices in the U.S. was up to $247,000.
  • Unemployment rate: The unemployment picture is very similar, with the unemployment rate in January 1998 (4.6%) coming in just below that of January 2017 (4.8%). 
  • Donald Duck, Goofy, and Mickey Mouse posing for a photo.

    Image source: Disneyland.

    Disney World ticket price: Heading to Disney (NYSE:DIS) World in Florida in 1998 would have cost $39.75 per adult, or $288 for an annual pass. Today, a regular ticket runs $115, with an annual pass setting a consumer back $779. Mind you, Disney just increased its theme-park ticket prices this week.
  • Wilshire 5000: The Wilshire 5000 market-cap-weighted index contained more than 7,500 companies as of July 1998, as private companies rushed to meet investor demand and go public. By Dec. 31, 2016, there were only 3,618 publicly traded companies in the Wilshire 5000.
  • 30-year fixed mortgage rate: At the beginning of January 1998, homebuyers would have been dealing with an average 30-year mortgage rate of 6.99%. Today, the 30-year rate is a more appealing 4.27%, per Bankrate.
  • One dozen eggs: Back in 1998, heading to the store for a dozen eggs would set you back $1.09. As of December 2016, the price for a dozen eggs had only risen to $1.38 (although that's down 50% from December 2015).
  • Slices of uncooked bacon.

    Image source: Pixabay.

    Bacon price per pound: Back in 1998, a pound of sizzling bacon would set a consumer back $2.96. According to the Bureau of Labor Statistics, a pound of sliced bacon was selling for $5.10 in December 2016. 
  • National debt: All the way back in 1998, the federal debt totaled "just" shy of $5.8 trillion. Within a matter of weeks, we're liable to cross the $20 trillion federal debt threshold.
  • Median household income: In 1998, median household income was $51,295, when adjusted for inflation. By 2015, the latest data available from Census data, median household income had risen to $55,775. 
  • U.S. postage stamp: A U.S. postage stamp increased to $0.33 in 1998, which compares with the $0.47 consumers pay for postage stamps today.

Prices change, but one aspect remains quite steady

A lot has changed over the past 19 years as the S&P 500's market cap has doubled in value. Yet despite these big changes, one factor remains firmly in place: Over time, the valuation of the stock market, and of high-quality companies, tends to increase.

Woman reading a financial newspaper.

Image source: Getty Images.

On average, the stock market returns about 7% per year, with dividend reinvestment factored in. This would put the stock market on track to double in value about once every decade. Understand, this is just an average, and as we saw between 1998 and 2017, it took a bit longer. Then again, culling data can be arbitrary. For example, the S&P 500 has risen more than threefold since its lows in 2009. If you had regularly been buying stocks without paying any attention to where the S&P 500 was, you'd likely have purchased companies that have increased multiple times in value since 2009.

According to data from Yardeni Research, the S&P 500 has undergone 35 stock market corrections of at least 10% since 1950, when rounded to the nearest whole number. In every single instance, the stock-market correction was wiped out by a bull-market rally. Sometimes it took weeks, months, and in rarer cases a few years, but when all was said and done, patient long-term investors were rewarded with a higher-marching stock market.

What's more, a study released by J.P. Morgan Asset Management found that holding the S&P 500 between January 1988 and November 2015 would have led to an average annualized return of 10.8%. However, missing just its 40 best days over this 27-year, roughly 7,000-day period would have reduced your average annual return to 2.9%!

The data doesn't lie. Holding high-quality companies over the long run, and buying companies at regular intervals, regardless of where the S&P 500 or other indexes are valued, is a winning strategy.

Sean Williams has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Apple and Walt Disney. The Motley Fool has the following options: long January 2018 $90 calls on Apple and short January 2018 $95 calls on Apple. The Motley Fool has a disclosure policy.