@jezrael 's solution is correct, but uses a deprecated alias for Seconds:

Deprecated since version 2.2.0: Aliases H, BH, CBH, T, S, L, U, and N are deprecated in favour of the aliases h, bh, cbh, min, s, ms, us, and ns.

df['Date'] = pd.to_datetime(df['Date']).dt.floor('s')