Before making the Bracha on counting the Omer one should know which day one is going to count.
If one has not yet counted the Omer, and somebody wants to know which day we're up to, you should tell them "yesterday was such-and-such".
If, instead, you replied "today is such-and-such" you may not be allowed to make a Bracha on that night's count, since you already counted.
However, you should still count "properly" (without a Bracha) since you have to mention the weeks as well.
Source: Kitzur Shulchan Aruch 120:3, 4
Wednesday, 3 Iyar 5775, 18th day of the Omer; 2 weeks and 4 days
Wednesday, April 22, 2015
לְכַתְּחִלָּה קֹדֶם שֶׁיְבָרֵךְ, צָרִיךְ שֶׁיֵדַע עַל מַה הוּא מְבָרֵךְ, דְּהַיְנוּ שֶׁיֵּדַע כַּמָּה יָמִים הוּא בַּסְּפִירָה, וּבְדִיעֲבַד אִם לֹא יָדַע וּפָתַח וּבֵרֵךְ עַל דַּעַת שֶׁיִּסְפּוֹר כְּמוֹ שֶׁיִּשְׁמַע מֵחֲבֵרוֹ, גַּם כֵּן יָצָא
Posted by Danny Schoemann at Wednesday, April 22, 2015