Hi @ells,
This Date table M function contains Offsets for several Date dimensions.
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These Offsets are evaluated each time the Date table query is refreshed. The basic principle behind them is that past periods are negative, future periods positive and current periods equal to 0.
This allows you to put an Between Offset Slicer on your report page OR in the Filter pane.
Without changing the current Slicer selection depicted above, as time progresses and we move into July it will automatically show months Feb-Jul.
I hope this is helpful.