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

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

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.