I need to know if i configure an analytic rule, with "TimeGenerated <= ago(1d)" inside the query, which time frame does it refer to when it is running?
the answer depends on your Log Analytics workspace data retention settings. For example, if you have retention configured for 30 days, then the query results will refer the time range between now-30d and now-1d.
You can always test the datetime information returned, one idea (or use arg_min or arg_max):
February 21, 2020