I cannot imagine having the spare time to invest in building date/time foundations and maintaining them through changes to DST timing and country/time zone changes.
The only crazier idea I can think of is implementing character encoding conversions myself.
Seriously.
Nobody needs a package for "left-pad", which is the most infamous example.
But there are a lot of areas where it's not a good use of your time to reinvent the wheel. There's no moral virtue in writing everything yourself. And popular packages have gone through a more bug-finding and bug-fixing than your personal code probably ever will.