The way I understand it is that the Total PPM error stated for crystals includes 10 years of aging.

Say you have a crystal with +/-50ppm tolerance. When you first install the crystal it will be very close to the specified frequency (+/-5ppm), and it's anticipated that in 10 years, the frequency will still be within the stated 50ppm @25°C.

Most crystals have an aging of less than 5-10 ppm per year. If you've come across a crystal that has +/-100ppm per year, throw it in the garbage.

For a recommendation, I'd still have to say the DS32KZ is the best for the price at less than $10.
http://pdfserv.maxim-ic.com/en/ds/DS32kHz-DS32KHZS.pdf
With an indoor application it will hold to +/- 2 ppm, which is 1 minute per year.
For outdoor apps, it's 7.5 ppm, or 4 minutes per year.

TCXO's for the CPU's osc at 10-20Mhz are more expensive, but some are really accurate. Like this one that holds to 0.28 ppm, or about 9 seconds/year. So after your 100 years it would only be off by 15 minutes. ($30US, 3.3V)
http://www.conwin.com/datasheets/tx/tx236.pdf

There are so many possible combinations of tolerance, compensation and time, it's difficult to give a direct answer. But for any of the possibilities, if you can come up with the anticipated PPM Error, this calculator will help to determine how much it would affect the long term timing of a clock.

Press the Recalculate button after changing the Freq or Error.
<table cellspacing="10"><tr><td valign="top"><OBJECT CLASSID="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" CODEBASE="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=6,0,40,0" WIDTH="400" HEIGHT="250" ><PARAM NAME="MOVIE" VALUE="http://www.pbpgroup.com/CrystalErr/CrystalPPM.swf"><PARAM NAME="PLAY" VALUE="true"><PARAM NAME="LOOP" VALUE="true"><PARAM NAME="QUALITY" VALUE="high"><EMBED SRC="http://www.pbpgroup.com/CrystalErr/CrystalPPM.swf" WIDTH="400" HEIGHT="250" PLAY="true" LOOP="true" WMODE="opaque" QUALITY="high" TYPE="application/x-shockwave-flash" PLUGINSPAGE="http://www.macromedia.com/go/getflashplayer"></EMBED></OBJECT></td><td valign="top">For normal crystals, the PPM tolerance is specified at 25°C. Temperature extremes in either direction can cause a 50ppm crystal to change as much as 100-300 ppm. Granted, those are extremes that would kill a human being, but somewhere in-between are the changes from summer to winter in an outdoor application.

The TCXO (Temperature Compensated Crystal Oscillator) will actively adjust the frequency according to the temperature, so that the frequency is always within the stated PPM tolerance. It usually does this by changing the voltage to a Varactor Diode, which changes the capacitance on the crystal. While it does compensate for temperature, the relative aging of the device remains the same as an uncompensated crystal.

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Happy Holidays!