There's related bug reports for scenarios where intervals are treated as a point in time.
It would be a good idea to come up with distinct and consistent presentations for points in time and intervals across the site. For points in time, we already use:
in [n] [unit of time]s
[n] [unit of time]s ago
[explicit date format]
Note that you need different formats, depending on whether the point in time is in the past or the future.
We need a separate format for intervals:
[a short time/less than a [minimum unit of time]]
[n] [unit of time]s
In this situation we are clearly looking at an interval of time, so it should follow the appropriate grammar:
Has been a member for: less than a [minimum unit of time].
Has been a member for: [n] [unit of time]s
Where a reasonable minimum unit of time is a day.
Alternatively, you could stick with the point in time format and use, "Has been a member since" instead of "Has been a member for":
Has been a member since: today
Has been a member since: [n] [unit of time]s ago
|Point in time format