Function format_timestamp 

pub fn format_timestamp(secs: f64, span_secs: f64) -> String

Formats a timestamp (seconds since the Unix epoch) as a calendar label, choosing granularity from the visible span_secs:

span	format
> ~2 years	YYYY
> ~2 months	YYYY-MM
> ~2 days	MM-DD
> ~2 minutes	HH:MM
otherwise	HH:MM:SS