It works perfectly fine.
If the error you are getting is in the Mendix Modeler Error(s) window then the issue you are facing is probably because your variable $Application/StartTimeNew is not a string.
On the other hand, if the error you are getting is in the runtime log, then it is probably an issue with your string (an empty string for example, or a string not in the specified format).
Please share the error you are getting to enable us to help you efficiently.
NOTE: Keep in mind that parseDateTime produces a DateTime variable, and when the date is not specified it will default to 1970-01-01. Just keep it in mind in case you’re using the function somewhere else to compare a string time with a DateTime variable.