Parse Cron Format Strings, Write Cron Format Strings and Caluclate Execution Dates.

1. Add crontab to your list of dependencies in mix.exs:

```elixir
def deps do
  [{:crontab, "~> 1.0.0-beta.4"}]
end
```

2. Ensure crontab is started before your application:

```elixir
def application do
  [applications: [:crontab]]
end
```

Please look into the Documentation for usage examples.