The labels shown for ucstart and ucend parameters in the list=usercontribs API query are as shown below.
Note that ucstart is the later timestamp and ucend is the earlier one (for example https://en.wikipedia.org/w/api.php?action=query&format=json&list=usercontribs&ucstart=2020-09-14T00%3A00%3A00.000Z&ucend=2019-09-14T00%3A00%3A00.000Z&ucuser=Huji returns results but https://en.wikipedia.org/w/api.php?action=query&format=json&list=usercontribs&ucstart=2019-09-14T00%3A00%3A00.000Z&ucend=2020-09-14T00%3A00%3A00.000Z&ucuser=Huji does not). The current label uses teh word "start timestamp" and "end timestamp" which implies the opposite, unless you know MW well-enough to relize that contribs are shown from later to earlier.
I suggest we change the labels to clearly say "earlier time" and "later time". Perhaps the top one can be changed to "The start timestamp to return form (i.e. revisions before this timestamp)" and the second one to "The end timestamp to return to (i.e. revisions after this timestamp)"