1)
Is "Yom Tza'akasi" only during the day?
1.
Radak: No, it includes the night. It says "b'Yom Hakosi Chol Bechor b'Eretz Mitzrayim" (Bamidbar 3:13), and Makas Bechoros was at night.
2)
Why does it say "va'Laylah Negdecha"?
1.
Rashi: Also at night, I am proper in front of You.
2.
Malbim: Time applies to people; what I screamed during the day does not exist at night. Time does not apply to Hashem; past, present and future are always in front of His eyes; there is no difference between them. Also at night, what I scream during the day is in front of Him.
