Series Round with fixed starting point

Hi all,

Let <tabSeries> be a set of time series values where the time points are always the first of month. Then

select series_round("ts",'interval 7 days',round_down) as "ts" from <tabSeries>

will produce a time series where the time points are always Saturdays before the first of the month.

Question: Is it possible to set up the series_round with Mondays? Why does the system take Saturdays? I would like to tell the system in series_round what should be the starting time point. Is this possible on the fly or do I have to persist a corresponding series table first?

Thanks,

Ingo