Calendars are arbitrary things. "December" and "2009" exist only because of ancient efforts by priests and mathematicians to contort the 12.36826639275 moon phase cycles that occur during each rotation of the tropical seasons into a unified time measurement system simple enough for people who don't like math. Why should it matter to stock investors how their holdings have performed since the start of an artificial period named for the two-faced Roman god Janus?

Mostly, it shouldn't. If we're nit-picking, though, investors should care because the tax man has come to care. Poor-performing stocks must be shed by December's end to yield tax benefits for the current calendar year. Even investors who don't have to trade for tax purposes should take note of the many other investors who do. In December, poor-performing stocks get dumped, making them cheaper than they otherwise would be. In many but not all cases, that causes this year's dogs to rebound during the first few weeks of next year, a well-known effect also named for that Roman god.

Investors should select stocks based on things like price, profits and dividends, not because of anything as fleeting or unreliable as the January effect. So please don't regard the three stocks below as my recommendations. They're simply year-to-date stinkers selected from the S&P 500 index. For each, I've highlighted the most obvious problems, along with the argument of at least one Wall Street analyst who says to buy.

MetroPCS Communications
YTD Price Decline: 49%

Pay-as-you-go cellphone plans used to be for drug dealers, kids and people with poor credit. Lately I'm wondering if I should sign up. The rise of mobile virtual network operators, or MVNOs -- companies that market calling plans while routing calls through the networks of larger operators -- has compressed per-minute charges from 25 cents to as little as 10 cents. There are no long-term contracts or thuggish termination fees. On one hand, if I use a pay-as-you-go plan, I have to give up my iPhone for something less sexy. On the other hand, my iPhone isn't great for making phone calls in certain places, like the capital of human civilization, New York City. The fierce price competition among MVNOs that is paying off for consumers also threatens the industry's future profitability, especially because traditional wireless operators, tired of ceding market share, are offering their own pay-as-you-go plans. Thus, the plunge in MetroPCS (PCS) shares this year. Most analysts who cover the stock don't recommend a purchase, but Romeo Reyes of Jefferies & Company does, citing "significant operating leverage" and growing free cash flow.

Citigroup
YTD Price Decline: 51%

Call me old-fashioned, but to me one of the chief jobs of a bank is to not go broke, being that it's the place that holds people's money. Citigroup (C) didn't do such a great job of that this year, requiring as it did a government bailout. Two things you need to know to decide whether to buy a stock are how much money the company will make and how much the company sells for. With Citigroup, it's difficult to discern either, because future profits depend in part on the ability of Americans to repay what they owe, and the company's future share price depends on whether it will have to further dilute its share count by issuing new shares to raise money. Matt O'Connor at Deutsche Bank sees the stock as a good deal because it trades below his estimate of year-end tangible book value (the liquidation value, roughly speaking).

Time Warner
YTD Price Decline: 20%

It wouldn't be fair to point out that Time Warner Cable (TWC) shares fell from a split-adjusted $64 and change at the start of the year to their current price of just over $41 without mentioning that the company paid a dividend of more than $10 a share earlier this year as part of a separation agreement with Time Warner (TWX). Then again, it's not fair that when my on-demand cable service stops working, I have to call up and listen to a chipper hold recording that insists, "We're committed to being the best," especially when, in many of its markets, Time Warner is the only (cable option, that is). With or without the dividend, the stock had a lousy 2009, in part because advertising was slow. Fiber-optic-laying phone companies and online video web sites are both long-term threats to the cable television business. Laura Martin of Needham & Company likes the stock because the company is paying down debt and looks likely to lavish growing dividends on shareholders. She has a point. If I had to pick one of this scruffy bunch, I suppose it would be this one.