Convert date format
[datestr,date_no] = thingspeak_convertdate(x,tgt,src)
Date string
Target string format if it is not default ("yyyy.MM.dd.HH.mm.ss")
Source string format if it is not default ("EEE MMM dd HH:mm:ss z yyyy")
Returned date string in new format
Returned date number
This is to convert the date string to other format