Starting the second night of Pesach men and women are obligated to count Sefiras HaOmer. One should ideally count after nightfall (tzeis hakochavim). However, some shuls (like ours) have the custom to recite Sefira after sunset, and some even after Plag HaMincha.
If one who attends services is confident that they will remember to say Sefira after nightfall, they should say Amen to the Chazzan/Rabbi’s blessing, not say a blessing, and count the Sefira while having in mind the following stipulation: “If I remember later on this should not be considered a counting. If I do not remember later on this should count.” Later in the evening, after nightfall, they should count Sefira with a blessing.
If one is not so confident that they will count later they should say the blessing at the earlier time and count. After nightfall, they should repeat the count without a blessing.
If one did not count for the entire evening, one should count during the following day without a blessing. If one missed counting for an entire day, they can no longer count Sefira with a blessing. However, there is still an obligation to continue to count every day without a blessing.