You can have an epoch and now you can measure times before the epoch. In terms of the range of values you can represent it transposes to just having the epoch f

You can have an epoch and now you can measure times before the epoch. In terms of the range of values you can represent it transposes to just having the epoch further in the past and using an unsigned type. So signed/unsigned it should not really matter except maybe for particular languages things work better if the type is either signed or unsigned. For example if you try to calculate the difference between two times maybe its better if the time type is signed to match the result type which is signed as well (not that it solves the problems with overflow).