Firmware version V1.3 has been released with some new functionality.  Changes:   Less memory consumption for both flash and dram. Estimated error displayed. Far more samples per measurement, 14500 instead of 3500. Calibration function added. 20 calibration points can be added.  Serial plotter of arduino IDE can be used to display measured power in time. See the manual for more details about the calibration.  

I have the older four digit version and I notice it is also about one second faster than WWV

The slope is 22.33mV per dB (22mV according to the datasheet).  If you measure 0,75V when connecting -20dBm input power then DBM_at_0V for that frequency will be: dBm_at_0V = 'Known input power' + ('measured voltage' / slope) -20dBm + (0,75V / 0.02233V) = 13.59 dBm Be sure to do measurements in the linear region (between -10dBm and -45dBm)  

You will be needing four of these kits: https://www.elektor.com/7-segment-display-150448-71?utm_source=MagazineSite&utm_medium=ProductLink led filaments are not included: buy a cheap led filament lamp and tear out ( carefully: they tend to break!!) the led filaments.

This code not tested as I haven't finished building clock yet.  Get closer by taking away two seconds in Time_To_Hex routine.  Delay of display by the remaing ~0.5 seconds is a bit more tricky - I'll come back to that after I finish build. ;------------------------------- ;TIME_TO_HEX = Make GPS time hex ;------------------------------- TIME_TO_HEX:    MOVLW    HIGH TIME_HEX_TABLE         MOVWF    TBLPTRH         MOVLW    LOW TIME_HEX_TABLE         MOVWF    TBLPTRL                  BCF    STATUS,C         MOVFF    HR_BCD,WREG         ADDWF    TBLPTRL         BTFSC    STATUS,C         INCF    TBLPTRH         TBLRD*         MOVFF    TABLAT,GPS_HR_HEX                 MOVLW    HIGH TIME_HEX_TABLE         MOVWF    TBLPTRH         MOVLW    LOW TIME_HEX_TABLE         MOVWF    TBLPTRL         BCF    STATUS,C         MOVFF    MIN_BCD,WREG         ADDWF    TBLPTRL         BTFSC    STATUS,C         INCF    TBLPTRH         TBLRD*         MOVFF    TABLAT,GPS_MIN_HEX         MOVLW    HIGH TIME_HEX_TABLE         MOVWF    TBLPTRH         MOVLW    LOW TIME_HEX_TABLE         MOVWF    TBLPTRL         BCF    STATUS,C         MOVFF    SEC_BCD,WREG         ADDWF    TBLPTRL         BTFSC    STATUS,C         INCF    TBLPTRH         TBLRD*         MOVFF    TABLAT,GPS_SEC_HEX ;START Andy Leitch ------------------ ;reduce seconds by 2                  call    decrement1sec         call    decrement1sec         return         ;subroutine to decrement seconds by 1                  decrement1sec:                 decf    GPS_SEC_HEX,F         btfss    GPS_SEC_HEX,7     ;check if seconds count is -1 (i.e. it went 0x00 to 0xFF)         return  ;...no, we are done         movlw    0x3B  ;.. yes, so set the seconds back to 59 and ..         movwf    GPS_SEC_HEX            decf    GPS_MIN_HEX,F     ;...decrement minutes         btfss    GPS_MIN_HEX,7    ; check if minutes became -1 (0xFF)         return  ;...no, we are done         movlw    0x3B  ;.. yes, so set the minutes back to 59 and ..         movwf    GPS_MIN_HEX            decf    GPS_MIN_HEX,F     ;...decrement hours         btfss    GPS_MIN_HEX,7    ; check if hours became -1 (0xFF)         return  ;...no, we are done         movlw    0x17  ;.. yes, so set the hours back to 23         movwf    GPS_HR_HEX    ;END Andy Leitch -------------         RETURN          ;----------------------------------- ;TIME_TO_BCD = Make hex PIC time BCD ;-----------------------------------