Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
First Section – the Positive Commandments
26. It is a positive commandment to count seven full weeks from the day the Omer was brought in the Temple.
As it says in Vayikra (Leviticus) 23:15, "And you shall count for yourself... seven full weeks." It is a commandment to count days as well as the weeks as it says in Vayikra 23:16, "You shall count fifty days." One should begin counting on the night of the 16th of Nissan. This counting requires a blessing but if one counted without a blessing, he has still fulfilled his obligation. One should count standing but if one counted while sitting, he has still fulfilled his obligation.
This applies in all places and at all times for men but not for women.