support decimal in date-time format

From [iso8601][1] wiki: If necessary for a particular application, the standard supports the addition of a decimal fraction to the smallest time value in the representation.

[iso8601][2] rfc states the following format with a decimal:

```
time-fraction  = ("," / ".") 1*DIGIT
time           = timespec-base [time-fraction] [time-zone]
iso-date-time = date "T" time
```

[1]: http://en.wikipedia.org/wiki/ISO_8601
[2]: http://www.ietf.org/rfc/rfc3339.txt