You declared it, but you didn't give it a value; you can't read a local variable until it is "definitely assigned". For a simple example:
DateTime myDate = DateTime.UtcNow; // is assigned
You don't have to give it a value right away... you can give it a value any time before you try to read it, including any branching etc that leaves no ambiguity that it has a value, for example:
DateTime myDate;
//....
if(condition) {
myDate = DateTime.UtcNow;
} else {
myDate = GetDateFromSomewhereElse();
}
Console.WriteLine(myDate);
For contrast, fields (class variables) are automatically initialized to their all-zero value, and are treated as "definitely assigned" from the object's creation.
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