Dynamic Fields on the Metrics Tab
Dynamic fields have different values depending on the context in which they are being used.
"Period Start" and "Period End" are both dynamic fields. When Period Start and Period Finish are being applied to a phase analysis, Period Start and Period Finish relate to the start and finish of the phase in question. When being used within the context of a ribbon, Period Start [_PeriodStart] and Period End [_PeriodFinish] relate to the start and end date of the ribbon.
In the context of Ribbons, the earliest start of the first activity in the Ribbon (time independent) is Period Start. Similarly, the end of the last activity in the Ribbon is Period Finish.
In the context of the Phase, Period Start is the start of a Phase and Period End is the end of the Phase.