How to find "next day date" (xx/xx/xx 00:00:00) from a given date & time value in Rules?
Let's see my example.

First: I created content type with two date fields: date1 and date2

Second: I created rule



Create Action 'Add variable' to store first date field value in your
variable
Create Action 'Set data value' and choose your second date field as
Data and your variable name as Value
Create Action 'Set data value' and choose your second date field as
Data and don't "Switch to data selector", simply set "+1 day
midnight" as Value


What's the point: when you r creating 'Set data value' action and choosing Date type you can see next description under Value field in form:


  The date in GMT. You may enter a fixed time (like 2015-08-16 14:05:10)
  or any other values in GMT known by the PHP strtotime() function (like
  "+1 day"). Relative dates like "+1 day" or "now" relate to the
  evaluation time.


If you google strtotime() php function you can simply understand what goes on.

"+1 day midinight":


"+" means add
"day" means what to add
"midnight" means "The time is set to 00:00:00"