Check whether a year is a leap year.
Leap Year
let( y, prop("Year"), y ? y / 400 % 1 == 0 || y / 4 % 1 == 0 && y / 100 % 1 != 0 : false )
Leap Year (From Date)
let( y, prop("Date").year(), y ? y / 400 % 1 == 0 || y / 4 % 1 == 0 && y / 100 % 1 != 0 : false )
Days This Year
lets( y, now().year(), l, y ? y / 400 % 1 == 0 || y / 4 % 1 == 0 && y / 100 % 1 != 0 : false, l ? 366 : 365 )