Wrappers for computing diff times

## Usage

```
diff_time(
x,
y,
method = c("secs", "mins", "hours", "days", "weeks", "months", "years", "dyears",
"wyears", "myears"),
tzx = NULL,
tzy = tzx
)
diff_time_days(x, y, ...)
diff_time_weeks(x, y, ...)
diff_time_hours(x, y, ...)
diff_time_mins(x, y, ...)
diff_time_secs(x, y, ...)
diff_time_months(x, y, ...)
diff_time_years(x, y, ...)
diff_time_dyears(x, y, ...)
diff_time_wyears(x, y, ...)
diff_time_myears(x, y, ...)
```

## Details

A few significant differences exist with these functions * The class
of the object returned is no longer `difftime`

(but does print) with the
`difftime`

method. This makes the exporting process easier as the data
will not have to be converted back to `numeric`

* `difftime()`

computes the
difference of `time1`

- `time2`

, but the inverse feels a bit more nature:
time difference from `x`

to `y`

* Additional units can be used (detailed
below) * Differences can be sensitive to time zones if time zones are
passed to the `tz`

parameter as a character vector

## Units

Units can be used beyond those available in
`base::difftime()`

. Some of these use assumptions in how units of time
should be standardized and can be changed in the corresponding options. Any
of these can be calculated with `base::difftime()`

through using `units = "days"`

but the `dtime`

class will print out with these specifications into
the console for less potential confusion.

- months
Months by number of days

`mark.days_in_month`

(defaults:`30`

)- years
Years by number of days

`mark.days_in_year`

(defaults:`365`

)- dyears
Years by number of days

`mark.days_in_year`

(defaults:`365`

) (same as`years`

)- myears
Years by number of days in a month

`mark.days_in_month`

(defaults:`30`

)- wyears
Years by number of weeks in a year

`mark.weeks_in_year`

(defaults:`52`

)

## Time zones

Time zones can be passed as either a numeric vector of
GMT/UTC offsets (the number of seconds from GMT) or as a character vector.
If the letter, these need to conform with values from `base::OlsonNames()`

.

A default timezone can be set with `options(mark.default_tz = .)`

. The
value can either be a numeric