set_graph_time(graph, time = NULL, tz = NULL)

# Create an empty graph
graph <- create_graph()

# Provide the new graph with a timestamp (if `tz`
# is not supplied, `GMT` is used as the time zone)
graph_1 <-
  graph %>%
  set_graph_time(time = "2015-10-25 15:23:00")

# Provide the new graph with a timestamp that is
# the current time; the time zone is inferred from
# the user's locale
graph_2 <-
  graph %>%
  set_graph_time()

# The time zone can be updated when a timestamp
# is present
graph_2 <-
  graph_2 %>%
  set_graph_time(tz = "America/Los_Angeles")