Comparison of a custom date field doesn’t work

Please help!
I have a custom date field named “_mem_start_date” and want to display future posts.
Tried
[loop type=post user_field=”_mem_start_date” value=”future-time”]
but comparison of this field doesn’t work, neither
user_field=”_mem_start_date” value=future nor
user_field=”_mem_start_date” value=future-time

Thank You