Function calculate_now 

pub fn calculate_now<P, T, F1, F2, O>(half_periods: F1, timer_value: F2) -> O
where
    T: TimerValue,
    O: From<P> + From<T> + TimerOps,
    F1: FnOnce() -> P,
    F2: FnOnce() -> T,

Calculates the current time from the half period counter and the timer value.

Arguments

• half_periods - A closure/function that when called produces the period counter value. If read from an atomic, can use Ordering::Relaxed.
• timer_value - A closure/function that when called produces the current timer value.