unique count the number of users in month regardless their frequency

Good day your EXCEL-lencies,

I have a signup record that expected to grow up to row 400. On the top, there's a monthly tally for reports. Column B to count the number of boxes taken and Column C for the users picked up the boxes.

Users should be accounted as one appearence in a month, regardless re-appearance in the same month or different month.

I haven't found the generic formula to uniquely count the number of users regardless their frequency in a month. Please advice. Thank you in advance.

NOTE:
Timings, box numbering format and usenames are randomly generated. Any similarities with real subjects will be coincidence only.