I don't think this is the best check to use, here.  ```isNaN()``` does not really map precisely to "is an integer".  My preference would be to have a pair of regular expressions for each allowed syntax here, so that we don't leak parts of the domain space...

i.e. ```/[0-9]+/``` -> current behaviour (epoch seconds), and ```/.... exact ISO8601 regex.../``` -> new behaviour, else bail out with `EINVAL`.