I presume the reason for this behaviour is the fact that Start/End Time in Outlook/Exchange is saved as a serial number. Is there any solution?
Are you creating an event with the Workflow? Is this in a SharePoint Calendar or are you using this with Outlook integration?
The end time is a date-time field. Are you supplying both values?
I found the solution after a hint from a collegue: It seems to be a SharePoint bug (which was described firstly in 2008 but in a programing context! See Link : https://social.msdn.microsoft.com/Forums/office/en-US/31e84d74-3ea8-44df-86dc-2dc62381ab3b/set-endda...
If you want to change the "End Time" field you have to update additionally the field "start Time" (and vice versa). Now SharePoint accepts the modification!