Add Date and Time to Files Written to an SD Card

If you are logging data to an SD card from an Arduino and all your files look like they were created in some cursed timeless dimension, this is the missing piece.

The short version is that FAT filesystems do not magically know the date and time. You have to provide a callback so the SD library can ask, “what time is it, then?”

What you need

• an SD card library that supports dateTimeCallback
• a real-time clock, or some other reliable source of date/time
• a faint sense of annoyance at seeing every file show the wrong timestamp

The important bit

The callback packs the current date and time into the FAT format expected by the library:

void dateTime(uint16_t* date, uint16_t* time) {
  *date = FAT_DATE(year, month, day);
  *time = FAT_TIME(hour, minute, second);
}

Then register it before writing files:

SdFile::dateTimeCallback(dateTime);

That is the whole trick. No secret handshake. Just a callback.

Where the time comes from

You still need to provide the values for:

• year
• month
• day
• hour
• minute
• second

In practice, that usually means reading from an RTC module such as a DS3231. If you are trying to fake this from millis(), you can get away with it for short-lived experiments, but it is not what I would call a noble long-term plan.

Why it matters

Correct timestamps make SD logging much easier to work with later:

• log files sort properly
• debugging becomes less archaeological
• you can tell whether the sensor actually ran today or last month

Common mistakes

• Registering the callback too late, after files are already being created.
• Feeding local variables that are not actually updated from the RTC.
• Forgetting that FAT timestamps have limited resolution, so “perfectly precise” is not on offer here.

This page is the slightly more civilised version for Future Me, who absolutely will forget the callback name again.