Converting human readable date to a unix timestamp
After parsing the time into numbers, you can use the standard C time library time.h. More specifically, use the mktime function to convert a struct tm into a time_t, and use difftime to get the seconds since the start of the Unix expoch:

#include &lt;time.h&gt;

void loop() {
    struct tm ltm = {0};
    ltm.tm_year = 2019 - 1900;
    ltm.tm_mon  = 11 - 1;
    ltm.tm_mday = 5;

    double epoch_seconds = difftime(mktime(&amp;ltm), (time_t)0);
    Serial.print("Remember, remember, ");
    Serial.print(epoch_seconds);
}


Note that in struct tm, the year starts at 1900 and the month starts at 0.