Why is every year the same number of days despite the gravity in the solar system?
Derived from this paper the interval between March equinoxes for 1989/1990 has been 4 minutes 28 seconds longer than the corresponding interval for 1987/1988.

So the length of a year can vary several minutes, e.g. by gravitational effects of other planets. For calendars a mean year is used. Atomic clocks run far more precise than the more or less periodic motion of Earth.

All planets aligned don't cause anything grave. A similar kind of sysygy has been 1982.