System Time



ESP32-C3 uses two hardware timers for the purpose of keeping system time. System time can be kept by using either one or both of the hardware timers depending on the application's purpose and accuracy requirements for system time. The two hardware timers are:

  • RTC timer: This timer allows time keeping in various sleep modes, and can also persist time keeping across any resets (with the exception of power-on resets which reset the RTC timer). The frequency deviation depends on the RTC Timer Clock Sources and affects the accuracy only in sleep modes, in which case the time will be measured at 6.6667 μs resolution.

  • High-resolution timer: This timer is not available in sleep modes and will not persist over a reset, but has greater accuracy. The timer uses the APB_CLK clock source (typically 80 MHz), which has a frequency deviation of less than ±10 ppm. Time will be measured at 1 μs resolution.

The possible combinations of hardware timers used to keep system time are listed below:

  • RTC and high-resolution timer (default)

  • RTC

  • High-resolution timer

  • None

It is recommended that users stick to the default option as it provides the highest accuracy. However, users can also select a different setting via the CONFIG_NEWLIB_TIME_SYSCALL configuration option.

RTC Timer Clock Sources

The RTC timer has the following clock sources:

  • Internal 136 kHz RC oscillator (default): Features the lowest Deep-sleep current consumption and no dependence on any external components. However, the frequency stability of this clock source is affected by temperature fluctuations, so time may drift in both Deep-sleep and Light-sleep modes.

  • External 32 kHz crystal: Requires a 32 kHz crystal to be connected to the XTAL_32K_P and XTAL_32K_N pins. This source provides a better frequency stability at the expense of a slightly higher (by 1 μA) Deep-sleep current consumption.

  • External 32 kHz oscillator at XTAL_32K_P pin: Allows using 32 kHz clock generated by an external circuit. The external clock signal must be connected to the XTAL_32K_P pin. The amplitude should be less than 1.2 V for sine wave signal and less than 1 V for square wave signal. Common mode voltage should be in the range of 0.1 < Vcm < 0.5xVamp, where Vamp stands for signal amplitude. In this case, the XTAL_32K_P pin cannot be used as a GPIO pin.

  • Internal 17.5 MHz oscillator, divided by 256 (about 68 kHz): Provides better frequency stability than the Internal 136 kHz RC oscillator at the expense of a higher (by 5 μA) Deep-sleep current consumption. It also does not require external components.

The choice depends on your requirements for system time accuracy and power consumption in sleep modes. To modify the RTC clock source, set CONFIG_RTC_CLK_SRC in project configuration.

More details about the wiring requirements for the external crystal or external oscillator, please refer to ESP32-C3 Hardware Design Guidelines.

Get Current Time

To get the current time, use the POSIX function gettimeofday(). Additionally, you can use the following standard C library functions to obtain time and manipulate it:


To stop smooth time adjustment and update the current time immediately, use the POSIX function settimeofday().

If you need to obtain time with one second resolution, use the following code snippet:

time_t now;
char strftime_buf[64];
struct tm timeinfo;

// Set timezone to China Standard Time
setenv("TZ", "CST-8", 1);

localtime_r(&now, &timeinfo);
strftime(strftime_buf, sizeof(strftime_buf), "%c", &timeinfo);
ESP_LOGI(TAG, "The current date/time in Shanghai is: %s", strftime_buf);

If you need to obtain time with one microsecond resolution, use the code snippet below:

struct timeval tv_now;
gettimeofday(&tv_now, NULL);
int64_t time_us = (int64_t)tv_now.tv_sec * 1000000L + (int64_t)tv_now.tv_usec;

SNTP Time Synchronization

To set the current time, you can use the POSIX functions settimeofday() and adjtime(). They are used internally in the lwIP SNTP library to set current time when a response from the NTP server is received. These functions can also be used separately from the lwIP SNTP library.

Some lwIP APIs, including SNTP functions, are not thread safe, so it is recommended to use esp_netif component when interacting with SNTP module.

To initialize a particular SNTP server and also start the SNTP service, simply create a default SNTP server configuration with a particular server name, then call esp_netif_sntp_init() to register that server and start the SNTP service.

esp_sntp_config_t config = ESP_NETIF_SNTP_DEFAULT_CONFIG("");

This code automatically performs time synchronization once a reply from the SNTP server is received. Sometimes it is useful to wait until the time gets synchronized, esp_netif_sntp_sync_wait() can be used for this purpose:

if (esp_netif_sntp_sync_wait(pdMS_TO_TICKS(10000)) != ESP_OK) {
    printf("Failed to update system time within 10s timeout");

To configure multiple NTP servers (or use more advanced settings, such as DHCP provided NTP servers), please refer to the detailed description of SNTP API in esp_netif documentation.

The lwIP SNTP library could work in one of the following sync modes:

  • SNTP_SYNC_MODE_IMMED (default): Updates system time immediately upon receiving a response from the SNTP server after using settimeofday().

  • SNTP_SYNC_MODE_SMOOTH: Updates time smoothly by gradually reducing time error using the function adjtime(). If the difference between the SNTP response time and system time is more than 35 minutes, update system time immediately by using settimeofday().

If you want to choose the SNTP_SYNC_MODE_SMOOTH mode, please set the esp_sntp_config::smooth to true in the SNTP configuration struct. Otherwise (and by default) the SNTP_SYNC_MODE_IMMED mode will be used.

For setting a callback function that is called when time gets synchronized, use the esp_sntp_config::sync_cb field in the configuration struct.

An application with this initialization code periodically synchronizes the time. The time synchronization period is determined by CONFIG_LWIP_SNTP_UPDATE_DELAY (the default value is one hour). To modify the variable, set CONFIG_LWIP_SNTP_UPDATE_DELAY in project configuration.

A code example that demonstrates the implementation of time synchronization based on the lwIP SNTP library is provided in the protocols/sntp directory.

Note that it is also possible to use lwIP API directly, but care must be taken to thread safety. Here we list the thread-safe APIs:


To set the local timezone, use the following POSIX functions:

  1. Call setenv() to set the TZ environment variable to the correct value based on the device location. The format of the time string is the same as described in the GNU libc documentation (although the implementation is different).

  2. Call tzset() to update C library runtime data for the new timezone.

Once these steps are completed, call the standard C library function localtime(), and it returns the correct local time taking into account the timezone offset and daylight saving time.

Year 2036 and 2038 Overflow Issues

SNTP/NTP 2036 Overflow

SNTP/NTP timestamps are represented as 64-bit unsigned fixed point numbers, where the first 32 bits represent the integer part, and the last 32 bits represent the fractional part. The 64-bit unsigned fixed point number represents the number of seconds since 00:00 on 1st of January 1900, thus SNTP/NTP times will overflow in the year 2036.

To address this issue, lifetime of the SNTP/NTP timestamps has been extended by convention by using the MSB (bit 0 by convention) of the integer part to indicate time ranges between years 1968 to 2104 (see RFC2030 for more details). This convention is implemented in lwIP library SNTP module. Therefore SNTP-related functions in ESP-IDF are future-proof until year 2104.

Unix Time 2038 Overflow

Unix time (type time_t) was previously represented as a 32-bit signed integer, leading to an overflow in year 2038 (i.e., Y2K38 issue). To address the Y2K38 issue, ESP-IDF uses a 64-bit signed integer to represent time_t starting from release v5.0, thus deferring time_t overflow for another 292 billion years.

API Reference

Header File

  • components/lwip/include/apps/esp_sntp.h

  • This header file can be included with:

    #include "esp_sntp.h"
  • This header file is a part of the API provided by the lwip component. To declare that your component depends on lwip, add the following to your CMakeLists.txt:

    REQUIRES lwip




void sntp_sync_time(struct timeval *tv)

This function updates the system time.

This is a weak-linked function. It is possible to replace all SNTP update functionality by placing a sntp_sync_time() function in the app firmware source. If the default implementation is used, calling sntp_set_sync_mode() allows the time synchronization mode to be changed to instant or smooth. If a callback function is registered via sntp_set_time_sync_notification_cb(), it will be called following time synchronization.


tv -- Time received from SNTP server.

void sntp_set_sync_mode(sntp_sync_mode_t sync_mode)

Set the sync mode.



sync_mode -- Sync mode.

sntp_sync_mode_t sntp_get_sync_mode(void)

Get set sync mode.


SNTP_SYNC_MODE_IMMED: Update time immediately. SNTP_SYNC_MODE_SMOOTH: Smooth time updating.

sntp_sync_status_t sntp_get_sync_status(void)

Get status of time sync.

After the update is completed, the status will be returned as SNTP_SYNC_STATUS_COMPLETED. After that, the status will be reset to SNTP_SYNC_STATUS_RESET. If the update operation is not completed yet, the status will be SNTP_SYNC_STATUS_RESET. If a smooth mode was chosen and the synchronization is still continuing (adjtime works), then it will be SNTP_SYNC_STATUS_IN_PROGRESS.


SNTP_SYNC_STATUS_RESET: Reset status. SNTP_SYNC_STATUS_COMPLETED: Time is synchronized. SNTP_SYNC_STATUS_IN_PROGRESS: Smooth time sync in progress.

void sntp_set_sync_status(sntp_sync_status_t sync_status)

Set status of time sync.


sync_status -- status of time sync (see sntp_sync_status_t)

void sntp_set_time_sync_notification_cb(sntp_sync_time_cb_t callback)

Set a callback function for time synchronization notification.


callback -- a callback function

void sntp_set_sync_interval(uint32_t interval_ms)

Set the sync interval of SNTP operation.

Note: SNTPv4 RFC 4330 enforces a minimum sync interval of 15 seconds. This sync interval will be used in the next attempt update time throught SNTP. To apply the new sync interval call the sntp_restart() function, otherwise, it will be applied after the last interval expired.


interval_ms -- The sync interval in ms. It cannot be lower than 15 seconds, otherwise 15 seconds will be set.

uint32_t sntp_get_sync_interval(void)

Get the sync interval of SNTP operation.


the sync interval

bool sntp_restart(void)

Restart SNTP.


True - Restart False - SNTP was not initialized yet

void esp_sntp_setoperatingmode(esp_sntp_operatingmode_t operating_mode)

Sets SNTP operating mode. The mode has to be set before init.


operating_mode -- Desired operating mode

void esp_sntp_init(void)

Init and start SNTP service.

void esp_sntp_stop(void)

Stops SNTP service.

void esp_sntp_setserver(u8_t idx, const ip_addr_t *addr)

Sets SNTP server address.

  • idx -- Index of the server

  • addr -- IP address of the server

void esp_sntp_setservername(u8_t idx, const char *server)

Sets SNTP hostname.

  • idx -- Index of the server

  • server -- Name of the server

const char *esp_sntp_getservername(u8_t idx)

Gets SNTP server name.


idx -- Index of the server


Name of the server

const ip_addr_t *esp_sntp_getserver(u8_t idx)

Get SNTP server IP.


idx -- Index of the server


IP address of the server

bool esp_sntp_enabled(void)

Checks if sntp is enabled.


true if sntp module is enabled

static inline void sntp_setoperatingmode(u8_t operating_mode)

if not build within lwip, provide translating inlines, that will warn about thread safety

static inline void sntp_servermode_dhcp(int set_servers_from_dhcp)
static inline void sntp_setservername(u8_t idx, const char *server)
static inline void sntp_init(void)
static inline const char *sntp_getservername(u8_t idx)
static inline const ip_addr_t *sntp_getserver(u8_t idx)



Aliases for esp_sntp prefixed API (inherently thread safe)


Type Definitions

typedef void (*sntp_sync_time_cb_t)(struct timeval *tv)

SNTP callback function for notifying about time sync event.

Param tv

Time received from SNTP server.


enum sntp_sync_mode_t

SNTP time update mode.



Update system time immediately when receiving a response from the SNTP server.


Smooth time updating. Time error is gradually reduced using adjtime function. If the difference between SNTP response time and system time is large (more than 35 minutes) then update immediately.

enum sntp_sync_status_t

SNTP sync status.


enum esp_sntp_operatingmode_t

SNTP operating modes per lwip SNTP module.