MSP430F5528 + RTC clock 32.768KHz

hi everybody
I need a RTC on my project with MSP430F5528. I have a crystal of 32.768KHz +/- 20 ppm connected to XT1 with internal capacitors (9pF). I use this crysal how source of ACLK and the ACLK how source of the timer B for the RTC.
 My problem is that RTC comes forward 1 seconds each 15 minutes. I put the ACLK on external pin for see your frequency with a oscillator and I see 32.810KHz instead of 32.768KHz. My point is: with +/-20 ppm I must see 32.768KHz +/- 0.00065KHz.
Please: someone has idea about this problem? RTC is critical for me.
Thanks very much. Best Regards