For various API timestamp-typed parameters, you can pass unusual values such as the empty string or "0" and it will be interpreted as meaning "now", which doesn't make very much sense except for the fact that it has always done that. If you really mean "now", you should pass that as the value instead.

action=edit even has to hack around this to avoid spurious edit conflicts if you do it for the 'basetimestamp' parameter. Ideally we'd make empty string and '0' be rejected as invalid timestamps, but first people have to stop passing them in.