But implementing a date picker UI isn't hard at all. If you have some particular extra requirement or a novel idea, go for it. That's also what front-end devs are for, reinvent UI for, I don't know, reasons.
Just, I certainly agree in this point, just leave the management of leap years to the date data type of the platform. Don't ever, not for a second (leap or not), think that += 24*60*60*1000 could be acceptable. But chances are you'd be far more aware of that wrongness while throwing together some novel date picker than anywhere else you might be tempted, where usually the peculiarities of calendar are far out of scope.
Just, I certainly agree in this point, just leave the management of leap years to the date data type of the platform. Don't ever, not for a second (leap or not), think that += 24*60*60*1000 could be acceptable. But chances are you'd be far more aware of that wrongness while throwing together some novel date picker than anywhere else you might be tempted, where usually the peculiarities of calendar are far out of scope.