Currently, if you create this scenario, you get the following error:

[info] - test_epochFillIn2 *** FAILED ***
[info] java.lang.Exception: Parse Error: Convert to Time (for xs:time): Cannot parse calendar from empty string
[info] Schema context: element.time23 Location line 2183 column 169 in file:/home/jchab/NGF/dfdl/daffodil-test/target/scala-2.9.2/test-classes/edu/illinois/ncsa/daffodil/section05/simple_types/SimpleTypes.tdml
[info] Data location was preceding byte 0
[info] UTF-8 text starting at byte 0 is: ()
[info] Data (hex) starting at byte 0 is: (0x)

However, if you create a calendar pattern of "." and your document is ".", the result is epoch time. This should be the result for the empty string as well.

see tests:
epochFillIn2
dateEpochFillIn2
in:
daffodil-test/src/test/scala-debug/edu/illinois/ncsa/daffodil/section05/simple_types/TestSimpleTypes2.scala