We have a great custom tool that allows you to change multiple Project Start Dates – so if you are interested in that … Contact Us.

However this question was around Changing Task Start Dates.
In Project, select all the Start Dates that you want to make the same date

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Then Right-Click and choose Information…  and select the start date and choose OK.

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You will see that the Start Dates change.

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Enjoy.
Jacques Goupil 
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