MINOR: sample: add a new "date" fetch to return the current date

MINOR: sample: add a new "date" fetch to return the current date

Returns the current date as the epoch (number of seconds since 01/01/1970).
If an offset value is specified, then it is a number of seconds that is added
to the current date before returning the value. This is particularly useful
to compute relative dates, as both positive and negative offsets are allowed.