Why is a time of 00:00 regarded as Null?

[Claus Dietrich <claus.dietrich@xxxxxxxxxx>]

One of my programs failed today. The reason was surprising:

A date consisting of a time component only and generated with an 
expression equivalent to this

dStartTime = cDate("00:00")

is regarded as Null. As my program checks the presence of a time 
expression with

if dStartTime then ...

it fails, if the time is 00:00, which is supposed to be a valid time.

If this is no bug, how can I check whether a date contains a valid 
time-only-component including 00:00 ?

Best regards

Claus