time: fix string representation of negative Duration

[hholst80]

Closes #20965

[hholst80]

@JalonSolov how do you think negative Duration should be represented as strings? just prefix with a '- ' like in debug string representation?

[JalonSolov]

@JalonSolov how do you think negative Duration should be represented as strings? just prefix with a '- ' like in debug string representation?

It should look like a negative number you typed in, so -10, not - 10 - that looks like you're trying to subtract 10 from something.

[spytheman]

Good work.