When editing a Dashboard, you can add Filters. (Top bar, below the name of the Dashboard)
One Filter can be used to apply to multiple Looks.
A window will pop-up in which you:
1. Give the filter a name
2. Describe the type of the filter. (e.g. Will it be a date that the user can alter, or a specific number?)
3. Set a default value that is used anytime the Dashboard is opened again.
4. Select all the Looks that you want the given parameter to apply to. For each Look you can then specify to which field within the Look it must apply to.
This way, you can make one filter that says - for example - 'in the past 6 months', and make all Looks in the Dashboard show data from the past 6 months, even if they are all based on a different Date column.
Then, you can change it to 'in the past 3 months', re-Run the Dashboard and see all Looks change to that filter.
I hope this has answered your question. Otherwise, I would suggest you try to make clearer what you want.