The before parameter should filter the set to only include phases that were active before the date specified (this means they started before the given date). The after parameter should filter the set to only include phases that were active after the date specified (this means they ended after the given date).
Here, only "Phase 1" would be selected because it started before the given date:
Time -------------------->
before
|
V
Phase 1: x x
Phase 2: x x
Phase 3: x x
Here, "Phase 2" and "Phase 3" would be selected because they ended after the given date
Time -------------------->
after
|
V
Phase 1: x x
Phase 2: x x
Phase 3: x x
These constraints can be used in conjunction to get phases that were active within a given window. This would select "Phase 1" and "Phase 2":
Time -------------------->
after before
| |
V V
Phase 1: x x
Phase 2: x x
Phase 3: x x