Normally, the TiVo clock will be set correctly using the Network Time Protocol (NTP). This is true whether or not you use a modem (PPP) or a network card (IP). However, in some cases, it may be more convienent to manually set the clock on the TiVo without using NTP.

From a TiVo bash session, type the following:

bash-2.02# export TZ=TIMEZONE
bash-2.02# date MMDDhhmmYY.ss

TIMEZONE should not be typed literally. It will likely be one of the following if you are in the US:

PST8PDT, MST7MDT, CST6CDT, EST5EDT
or PST, MST, CST, EST if you don't use daylight savings in your area.
(please use the correct Timezone setting here or set your time according to GMT rather than local time)

MM = 2 digit month
DD = 2 digit day
hh = 2 digit hour (24-hour format)
mm = 2 digit minute
YY = 2 digit year (optionally, 4 digits long)
ss = 2 digit seconds

Choose the right date/time and set it accordingly. This will set the proper time on your TiVo.