Is your feature request related to a problem? Please describe.
For now there's no explicit validation for values. Meaning that something like this will work just fine:

(take 10 (cron->dt "10-67 9 * * *"))

Invalid values (minutes 60 to 67) will be ignored.

Same works for:

• cron->text
• cron->map

Describe the solution you'd like
I think we need to:

1. Fail fast at the moment when string values are parsed, not at the moment of generating
2. Apply validation to all entrypoint functions
3. Have a separate function for cron validation, e.g. (cron-valid? [s])

Reproduce:

> (cron-validate "Mon-Fri)
Execution error (NullPointerException) at java.util.regex.Matcher/getTextLength (Matcher.java:1770).
Cannot invoke "java.lang.CharSequence.length()" because "this.text" is null

We need to catch base exception to handle the case

It would be cool to generate a string explaining in English a given crontab, e.g.

1. */10 * * 1-6 Mon-Fri => at every 10th minute past every hour on every day of week from Monday through Friday in every month from January through June
2. 0,15,30 9,12,21 */5 * * => at minute 0, 15, 30 past hour 9, 12, 21 on every 5th day of month
3. 0,7,15,*/12,*/14,57-59 * * * * => at minute 0, 7, 15, every 12th minute, every 14th minute, and every minute from 57 through 59
4. * * 1,3-7,*/10 * 1,2,*/3,5-7 => at every minute past every hour on day of month 1, every day of month from 3 through 7, and every 10th day of month or on Monday, Tuesday, every 3rd day of week, and every day of week from Friday through Sunday

So, the format is roughly:

at [concrete value(s), range(s), frequency(ies)][minute] 
past [concrete value(s), range(s), frequency(ies)][hour] 
on [concrete value(s), range(s), frequency(ies)][day of month] or [concrete value(s), range(s), frequency(ies) [day of week]
in [concrete value(s), range(s), frequency(ies)][month]

Values concatenations:

• concrete values a,b,c,d => a, b, c, d
• ranges 1-5 => from 1 though 5; Mon-Fri => from Monday through Friday
• frequencies */5 => every 5th