Something like this?
sourcetype=csv | epoch = strftime(Last_Log_In, "%Y-%m-%d %H:%M:%S")
| bin span=1d epoch | where relative_time(now(), "-30d@d") <= epoch
| stats count as Log_In_Counts by epoch | fieldformat epoch = strftime(epoch, "%F")
Row 1 grabs your data and converts your string to an epoch date, row 2 groups that date by day and filters for last 30 days, row 3 runs your counting report and formats the epoch as a user-readable date.
