做了一个30秒定时器，但它只能停在30秒不会倒计时，帮我看看是哪里出了问题，谢谢！
描述清楚功能和问题，从取加法十进位逻辑看是不成立的https://blog.csdn.net/xianfajushi/article/details/121943914?ops_request_misc=%257B%2522request%255Fid%2522%253A%2522165710954116781435472768%2522%252C%2522scm%2522%253A%252220140713.130102334.pc%255Fblog.%2522%257D&request_id=165710954116781435472768&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2~blog~first_rank_ecpm_v1~rank_v31_ecpm-1-121943914-null-null.185^v2^control&utm_term=192&spm=1018.2226.3001.4450
