Try something like the following. You can put this into a custom function and everytime you get the calendar string assign it to D$ and then call this function:
D$ = "10 MAY 2015 22:02:21 CEST"
IF LEN(D$) = 25 ' response with standard format of 25 characters
SETLCD 1,1,D$
DATE[3] = VAL(MID$(D$,1,2))
C$ = MID$(D$,4,3)
IF STRCMP(C$,"JAN")=0 DATE[2] = 1
ELSEIF STRCMP(C$,"FEB")=0 DATE[2] = 2
ELSEIF STRCMP(C$,"MAR")=0 DATE[2] = 3
ELSEIF STRCMP(C$,"APR")=0 DATE[2] = 4
ELSEIF STRCMP(C$,"MAY")=0 DATE[2] = 5
ELSEIF STRCMP(C$,"JUN")=0 DATE[2] = 6
ELSEIF STRCMP(C$,"JUL")=0 DATE[2] = 7
ELSEIF STRCMP(C$,"AUG")=0 DATE[2] = 8
ELSEIF STRCMP(C$,"SEP")=0 DATE[2] = 9
ELSEIF STRCMP(C$,"OCT")=0 DATE[2] = 10
ELSEIF STRCMP(C$,"NOV")=0 DATE[2] = 11
ELSEIF STRCMP(C$,"DEC")=0 DATE[2] = 12
ENDIF
DATE[1] = VAL(MID$(D$,8,4)) ' Get the Year
TIME[1] = VAL(MID$(D$,13,2)) '
TIME[2] = VAL(MID$(D$,16,2))
TIME[3] = VAL(MID$(D$,19,2))
SETLCD 4,1, "RTC Updated"
ELSE
SETLCD 4,1, "Fail to update RTC"
ENDIF
ENDIF
