Considering I wrote that little piece of an algorithm you posted, in a way yes. That will calculate timestamps for the previous day, but don't confuse that with the last 24 hours. The last 24 hours would simple be the current timestamp - 86401.

What you have pasted will generate two timestamps: $lts (lower timestamp) will be equal to 00:00:00 the previous day, and $uts (upper timestamp) equal to 23:59:59 the previous day.

These timestamps can be used in a MySQL SELECT statement for comparrison with the dateline member or such things as posts, threads, polls.