diff --git a/pkg/fatfs/fatfs_diskio/mtd/mtd_diskio.c b/pkg/fatfs/fatfs_diskio/mtd/mtd_diskio.c
index 9a5bf76a57..e0df082dd6 100644
--- a/pkg/fatfs/fatfs_diskio/mtd/mtd_diskio.c
+++ b/pkg/fatfs/fatfs_diskio/mtd/mtd_diskio.c
@@ -213,7 +213,7 @@ DWORD get_fattime(void)
     /* bit 31:25 Year origin from 1980 (0..127) */
     uint8_t year = time.tm_year + RTC_YEAR_OFFSET - FATFS_YEAR_OFFSET;
     uint8_t month = time.tm_mon + 1;        /* bit 24:21 month (1..12) */
-    uint8_t day_of_month = time.tm_mon + 1; /* bit 20:16 day (1..31) */
+    uint8_t day_of_month = time.tm_mday;    /* bit 20:16 day (1..31) */
     uint8_t hour = time.tm_hour;            /* bit 15:11 hour (0..23) */
     uint8_t minute = time.tm_min;           /* bit 10:5 minute (0..59) */
     uint8_t second = (time.tm_sec / 2);     /* bit 4:0 second/2 (0..29) */