At least in my mobile app (Android), I have a task sheduled to 15:45. Unhappily, it still shows time "14:45" even if I rechange it to 15:45.
I have the same problem too! :(
Due to daylight savings time, which is this Sunday, issues such as this can happen, but should be resolved after Sunday. Please let us know if they continue after such time.
If so, please make sure that Todoist's settings, and any devices you use are set to the same timezone.