Let’s say that your time value is in cell A1 then in A2 you can put:
=A1*1000*60*60*24
or simply:
=A1*86400000
What I am doing is taking the decimal value of the time and multiply it by 1000 (milliseconds) and 60 (seconds) and 60 (minutes) and 24 (hours).
You will then need to format cell A2 as General for it to be in milliseconds format.
If your time is a text value then use:
=TIMEVALUE(A1)*86400000
UPDATE
Per @dandfra’s comment this solution may not work in the Italian version of Excel.