cmp_day(d1: Date, d2: Date): int (-1, 0, or 1)

compares two dates, granular to the day, to determine if they are on the same day, or if one is later than the other.

• If d2 comes AFTER d1, returns 1
• if d2 comes BEFORE d1, returns -1
• if d2 comes ON THE SAME DAY as d1, returns 0.