Function parse_duration 

pub fn parse_duration(d: &str) -> Result<Duration, String>

Parses the &str argument as a Duration Returns Ok(Duration) if successful, or Err(String).

Format is defined as follows: <integer> <unit>

• whitespace between the integer and unit is optional
• integer must be positive (>= 0)
• unit can be one of:
  • ms / millis / millisecond / milliseconds
  • s / sec / secs / second / seconds
  • m / min / mins / minute / minutes
  • h / hour / hours
  • d / day / days