你是plc中编程？

plc中建立db，db总建立dtl格式的变量mydatetime，mydatetime1，mydatetime保存当前值，mydatetime1保存7天前的日期

IF "mydata".mydatetime.DAY >7THEN

    "mydata".mydatetime1.YEAR := "mydata".mydatetime.YEAR;

    "mydata".mydatetime1.MONTH := "mydata".mydatetime.MONTH;

    "mydata".mydatetime1.DAY := "mydata".mydatetime.DAY - 7;//如果日期大于7,则日期减去7

ELSIF  "mydata".mydatetime.MONTH >1 THEN

    "mydata".mydatetime1.YEAR := "mydata".mydatetime.YEAR;

    "mydata".mydatetime1.MONTH := "mydata".mydatetime.MONTH - 1;//否则如果月份大于1,则月份减1

    IF "mydata".mydatetime1.MONTH = 1 OR "mydata".mydatetime1.MONTH = 3 OR "mydata".mydatetime1.MONTH = 5 OR "mydata".mydatetime1.MONTH = 7 OR "mydata".mydatetime1.MONTH = 8 OR "mydata".mydatetime1.MONTH = 10 OR "mydata".mydatetime1.MONTH = 12 THEN

        "mydata".mydatetime1.DAY := 31 + ("mydata".mydatetime.DAY - 7);//如果如果减后的月份是大月,则日期为当前日期的差加上上一个月的日期值(30)

    ELSIF  "mydata".mydatetime1.MONTH<>2 THEN

        "mydata".mydatetime1.DAY := 30 + ("mydata".mydatetime.DAY - 7);//如果是上个月小月,且月份不是2,则日期为当前日期的差加上上一个月的日期值(31)

    ELSIF("mydata".mydatetime.YEAR MOD 400=0)OR (("mydata".mydatetime.YEAR MOD 4=0) AND ("mydata".mydatetime.YEAR MOD 100<>0) )THEN//判断是否闰年

        "mydata".mydatetime1.DAY := 29 + ("mydata".mydatetime.DAY - 7);//如果上一个月是2月,且为闰年,则日期为当前日期的差加上上一个月的日期值(29)

    ELSE

        "mydata".mydatetime1.DAY := 28 + ("mydata".mydatetime.DAY - 7);//如果上一个月是2月,且为平年,则日期为当前日期的差加上上一个月的日期值(28)

    END_IF;

ELSIF "mydata".mydatetime.MONTH =1 THEN//如果当前月份是1月且日期小于7

    "mydata".mydatetime1.MONTH := 12;//则上一个月是12月

    "mydata".mydatetime1.YEAR := "mydata".mydatetime1.YEAR - 1;//年为当前年份减1

    "mydata".mydatetime1.DAY := 31 + ("mydata".mydatetime.DAY - 7);//日期为当前日期的差加上上一个月的日期值(31)

END_IF;