 Use the correct zero value for `time.Time` throughout the codebase

The correct zero value for `time.Time` is `time.Time{}` instead of `time.Unix(0, 0)`. Using the latter can cause hard to find bugs since `func (Time) IsZero()` would return `false` when called on `time.Unix(0, 0)`. Details here: https://stackoverflow.com/questions/23051973/what-is-the-zero-value-for-time-time-in-go