Add datetime in MySQL

Example: plus datetime 31 days;

• update table_name set datetime_field = ADDDATE(datetime_field, 31);

Example: minus datetime 31 days;

• update table_name set datetime_field = ADDDATE(datetime_field, -31);

Calculate DPI for Monitor

Use this formula.
dpi = sqrt(resolutionWidth ^ 2 + resolutionHeight ^ 2) / inches