B2公式下拉填充：
=TEXT(A3-A2,"[s];0")
或：
=TEXT(A3-A2,"[s];!0")
还可加入空白判断：
=IF(A3="","",TEXT(A3-A2,"[s];0"))
或另一种空白判断：
=IF(A2&A3="","",TEXT(A3-A2,"[s];0"))
