Depending on how precise you want the numbers, you may want to round the calculation to zero decimal places

=ROUND(A2*86400,0)

Excel does very precise calculations, but multiplying a time value by 86400 can (for some values) result in a decimal being calculated rather than a whole number of seconds.

For example, 02:10:25 when multiplied by 86400 returns 7824.99999999995. If you were to compare the calculation to a value of 7825 (the actual number of whole seconds in 02:10:25), the two would not match. Rounding the calculation to zero decimal places resolves the issue.