/etc/nagios3/conf.d/timeperiods_nagios2.cfg is in nagios3-common 3.5.1-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# timeperiods.cfg
###############################################################################
# This defines a timeperiod where all times are valid for checks, 
# notifications, etc.  The classic "24x7" support nightmare. :-)
define timeperiod{
        timeperiod_name 24x7
        alias           24 Hours A Day, 7 Days A Week
        sunday          00:00-24:00
        monday          00:00-24:00
        tuesday         00:00-24:00
        wednesday       00:00-24:00
        thursday        00:00-24:00
        friday          00:00-24:00
        saturday        00:00-24:00
        }
# Here is a slightly friendlier period during work hours
define timeperiod{
        timeperiod_name workhours
        alias           Standard Work Hours
        monday          09:00-17:00
        tuesday         09:00-17:00
        wednesday       09:00-17:00
        thursday        09:00-17:00
        friday          09:00-17:00
        }
# The complement of workhours
define timeperiod{
        timeperiod_name nonworkhours
        alias           Non-Work Hours
        sunday          00:00-24:00
        monday          00:00-09:00,17:00-24:00
        tuesday         00:00-09:00,17:00-24:00
        wednesday       00:00-09:00,17:00-24:00
        thursday        00:00-09:00,17:00-24:00
        friday          00:00-09:00,17:00-24:00
        saturday        00:00-24:00
        }
# This one is a favorite: never :)
define timeperiod{
        timeperiod_name never
        alias           Never
        }
# end of file
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