It's true that stop watch utilities for Windows Phone 8.1 were ten a penny. But Stopwatch Yokto is different in that it's a full Windows 10 UWP, works fine even when it's not on-screen, and also has surprisingly sophisticated export facilities (for a stopwatch!)
Simple and useful stopwatch. With sound and vibration on the button click. Keeping measures in memory.
Work in background
Here's Stopwatch Yokto in action:
Responsive start/stop and lap controls, with lap times listed in a scrolling pane; (right) choosing to filter the timing history according to date and/or keyword.
Timing logs can be renamed as needed and (right) even exported in CSV format for examination in Excel or similar.
I tested the background operation and it worked just fine. So you can be timing a friend going round a track (etc.), stop to take a phone call or check your Calendar, and when you switch back to Stopwatch Yokto it's still tracking time perfectly. A nice little freeware UWP for all Windows 10 devices.