Make a Time of one day

Am I doing something wrong here?

Time oneDay {0, 0, 1, 0, 0};
Time alsoOneDay {1000 * 60 * 60 * 24};
DBG("One day is \t" << oneDay.toMilliseconds());
DBG("One day is also " << alsoOneDay.toMilliseconds());
jassert(oneDay == alsoOneDay);

Output:

JUCE v7.0.3
One day is      -62167200148000
One day is also 86400000
JUCE Assertion failure in Main.cpp:11

Have a look at RelativeTime.
Your first example is one day after the birth of Jesus:

Time (int year, int month, int day, int hours, int minutes, int seconds=0, int milliseconds=0, bool useLocalTime=true) noexcept

Your second example is one day after 1.1.1970:

Time (int64 millisecondsSinceEpoch) noexcept

Ahaaaaa, thank you!