A dashboard's dates control has two modes.
As of date: pick one day. Each widget keeps its own time period, counted back from that day. For example, a "last 30 days" chart shows the 30 days before the date you picked. Use this to see the dashboard as it looked on a given day.
Date range: pick a start and an end. Every widget uses that same range. Use this to compare everything over one period.
By default a dashboard shows Yesterday, as an as-of date in your company's time zone.
Change the dates
Open the dates control at the top of the dashboard.
Choose As of date or Date range.
Pick a date or a range, or use Quick select for options like End of last month or Last 30 days.
Select Apply.
[Image: The dates control open, with the two modes and the Quick select list]
The dates are part of the page address, so if you copy the link, the person you send it to sees the same dates. Reloading the page without them brings the dashboard back to its default.