Add year to Date object:
Multiply by 12 and add that many months (see below)

Subtract year from Date object:
Multiply by 12 and add that many months (see below)

Add month to Date object:
new_date = date >> 1

Subtract month from Date object:
new_date = date << 1   (or *add* a negative int)

Add day to Date object:
new_date = date + 1

Subtract day from Date object:
new_date = date - 1

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