This is a mod that replaces the normal pause screen when you're outside a level with a new screen that shows your best times for each star, along with combined times for the current course and the entire game. There's a level timer (that can be toggled on/off) and a separate screen for Bowser stars/keys and castle secret stars (instead of grouping them together like the original game), and you can delete times for a single course with a button combination.

Times are saved to EEPROM, so this even works on an actual N64 ROM... at least in theory; an N64 version is included in the download, but it's kinda buggy and doesn't actually save times when you turn the game off.

WARNING: Old saves are INCOMPATIBLE with this mod. In other words, your non-time-trial-mod save WILL BE DELETED if you apply this patch!!! Sorry, but it's a side effect of having to increase the EEPROM size to fit the times. Just make sure you move/rename your current save before playing this! Or, there is a workaround; this mod is compatible with the TEXTSAVES build option, so you can make a textsave of your current build, then compile this mod (still using TEXTSAVES), and it will import your old save. You'll have to get a star, save, then reset the game for an old save to acknowledge time trials... or you can add this to your save file below sound_mode:

time_trial_times = 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
time_trial_total_time = 0

One more thing: this is intended mainly for casual players; only individual stars/keys are timed, not the full run (use a stopwatch for that). Also, this doesn't check for cheats or any other modifications, so be honest when you show off your times :)

time_trials.zip

An updated version made by PeachyPeach can be found here, which adds ghost racing, external saves (making text saves unnecessary), and several QOL upgrades.