Is the currentdate in Worflow designer a static one

The behavior (Yesterday I received an email from "if" condition. Today I received an email from "else" condition.) that you have received is correct!

To make it clear, Let's review your above workflow steps,

• You are checking the start time of the current item that initiates the workflow if it is equal today or not.
• Then pause one hour.
• Then go to back to check again.

All the above steps have been only applied on an item that initiates this workflow instance whether in creating or changing or even manually.

So yesterday the start time of the current item was equal today so you have received an email from if condition and Today the start time of the current item that initiates this workflow becomes is not equal today so you have received from the else condition.

So you should be aware of every item in the list has its own workflow instance, and the workflow instance will be only applied to the item that initiates it, not for all items in the list.

In fact, I don't know what's your scenario to do your workflow like this but I think this is not a good way to build your workflow without End Workflow action!! With this way will lead to infinity workflow instance for each item in your list.