Generates a signal line based on the specified time range. The line will be 1
within the range and 0 for all other times. The format of the start and end times
is the same as the start and end time parameters on the Graph
API. If the time zone is not explicitly specified, then the value from the tz
variable will get used. The default value for the tz variable is the primary
time zone used for the graph.
The following named times are supported for time spans:
Graph start time.
Graph end time.
Start time for the span, can only be used for the end time.
End time for the span, can only be used for the start time.