If you are viewing your calendar in a timezone that is not your anchor timezone - e.g. you set up your working hours in the UK and are in the USA on holiday - you may find that in some weeks your working times will move by one hour.
This is the effect of the fact that the application of daylight savings time (or summer time in the UK) does not happen at the same time in every country and in some locations never happens.
So as an example if you had working times set up in the UK of 13:00 to 17:00 on a Monday, these would normally be shown as 08:00 to 12:00 in New York. However between early March (e.g. from 11th March in 2019) to the end of March the working times in New York would move to 09:00 to 13:00. They would then revert back to 08:00 to 12:00 from early April onwards.
If you move to another country you can change your working hours to link to your new timezone so they don't change in your location, however clients may see different times at different times of the year if they are not located in your timezone.