[WIP] Make seconds optional

[Dineshs91]

Any ideas on how to achieve this ?

Issue #13
Importantly, expressions that do not include seconds but which do include years would be illegal.

For this to work, I guess we have to parse the last part of the cron expression and check if it is an year and err out if seconds is not included.

I am wondering why this expression works 0/30 * * * * and this doesn't */30 * * * *.

[zslayton]

Importantly, expressions that do not include seconds but which do include years would be illegal.

Such an expression would be illegal as input but the parser would not be expected to detect that case with certainty.

If the expression had 6 fields, the expectation would be that the sixth field was the rightmost and represented years, not the leftmost and representing seconds. There will be some cases where the parser would successfully read the expression but be unaware that the user mistakenly wanted the sixth field to represent seconds (for example, * * * * * *).

[zslayton]

Closing this out as part of a cleanup. Feel free to re-open it if you pick it back up.