Are you using a Calendar JForm field for the “now” date? I had the same issue and found “now” works if you use the Calendar Picker field instead. Which is a shame as it’s not as attractive as the JForm version. Not sure what the difference is.
I'm no coder, but I can't see any difference between the JForm and the Calendar Picker field types. Both use datetime storage in the SQL tables and according to all the research I've done on JForm it should accept "now" as a default value. I've even looked at the PHP file in libraries/joomla/forms/fields but nothing jumps out (perhaps not surprising as I don't understand much about PHP!). Sorry.