From: Julian date or floating-point year
To:   Kalendar date or Julian date or floating-point year

The conversion option is -j

Remember:
      Add option i for floating-point year input 
      Add option o for Julian date or floating-point year output
      Add option y for floating-point year output

$ echo 2447311.0 | JDC -j -k3 -x -i -,
1988,05,29,12,00,00  

$ echo 1988.1 x | JDC -ji -k3 -x -F',6' -i
1988 02 06 12 36 00  x

$ echo 1988.1 x | JDC -jio -k3 -F',6' -i
47197.525000  x

$ echo 1988.1 | JDC -jio -k3 -F',6' -x -i
2447198.025000

$ echo 2447198.025000 x | JDC -j -k3 -F',15' -x -i
1988 02 06 12 36 00 x

$ echo 2447198.025000 x | JDC -joy -k3 -F',15' -x -i
1988.1000000000 x


From: Kalendar form
To:   Floating-point form (Julian or year)

$ echo 1995 jan 1 12 0 0  | JDC -i -k3 -dy 
1995.0020533881

$ echo 1995 jan 1 12 0 0  | JDC -i -k3 -d
49718.500000

$ echo 1995 jan 1 12 0 0  | JDC -i -k3 -x -d
2449719.000000

$ echo 1995 jan 1 12 0 0  | JDC -i -k3 -r -d
49719.000000

$ echo 1995 jan 1 12 0 0  | JDC -i -k3 -m -d
49718.500000

$ echo 1995 jan 1 12 0 55 | JDC -i -k3 -m -d
49718.500637

$ awk 'BEGIN{for (y=1993;y<1997;y++){print y,y}}' | JDC -i -k3 -F,5 -ji
1992 12 31 06 00 00 1993
1993 12 31 12 00 00 1994
1994 12 31 18 00 00 1995
1996 01 01 00 00 00 1996

$ awk 'BEGIN{for (y=1993.033;y<1995;y+=0.083333){printf "%8.2f %8.2f\n", y,y}}' | JDC -i -k3 -F,8 -N -ji
1993 JAN 11 04 58 48   1993.03
1993 FEB 13 01 55 12   1993.12
1993 MAR 14 07 12 00   1993.20
1993 APR 12 12 28 48   1993.28
1993 MAY 15 09 25 12   1993.37
1993 JUN 13 14 42 00   1993.45
1993 JUL 12 19 58 48   1993.53
1993 AUG 14 16 55 12   1993.62
1993 SEP 12 22 12 00   1993.70
1993 OCT 12 03 28 48   1993.78
1993 NOV 14 00 25 12   1993.87
1993 DEC 13 05 42 00   1993.95
1994 JAN 11 10 58 48   1994.03

# Make a monthly table
$ set from=`JDC -k1 -dy 1996,6,30,12` to=`JDC -dy 1998,4,30`
$ awk -v f=$from -v t=$to \
      'BEGIN{for(y=f;y<t;y+=0.0833333){printf "%8.2f %8.2f\n",y,y}}' |\
       JDC -i -n -F,9 -ji
1996 jul 01  1996.50
1996 jul 30  1996.58
1996 aug 29  1996.66
1996 sep 30  1996.75
1996 oct 30  1996.83
1996 nov 28  1996.91
1996 dec 31  1997.00
1997 jan 29  1997.08
1997 feb 27  1997.16
1997 apr 01  1997.25
1997 apr 30  1997.33
1997 may 30  1997.41
1997 jul 01  1997.50
1997 jul 31  1997.58
1997 aug 29  1997.66
1997 oct 01  1997.75
1997 oct 30  1997.83
1997 nov 28  1997.91
1997 dec 31  1998.00
1998 jan 29  1998.08
1998 feb 27  1998.16
1998 apr 01  1998.25
       
# GMT pstext 
$ awk -v j=55930 \
   'BEGIN{for(i=0;i<100;i++)\
         {printf "%18.8f %s\n", i/1440+j," -0.5 12 0 0 5"}}' |\
  JDC -j -m -K35 -i -F'(f18.0),35' -k3 -:
55930.00000000  -0.5 12 90 0 5 2012-01-04 00:00:00
55930.00069444  -0.5 12 90 0 5 2012-01-04 00:01:00
55930.00138889  -0.5 12 90 0 5 2012-01-04 00:02:00
55930.00208333  -0.5 12 90 0 5 2012-01-04 00:03:00
55930.00277778  -0.5 12 90 0 5 2012-01-04 00:04:00
55930.00347222  -0.5 12 90 0 5 2012-01-04 00:05:00
55930.00416667  -0.5 12 90 0 5 2012-01-04 00:06:00
55930.00486111  -0.5 12 90 0 5 2012-01-04 00:07:00
55930.00555556  -0.5 12 90 0 5 2012-01-04 00:08:00
55930.00625000  -0.5 12 90 0 5 2012-01-04 00:09:00
55930.00694444  -0.5 12 90 0 5 2012-01-04 00:10:00
...

# ... with increment 10 minutes:
echo "2012 01 04 -0.5 12 90 0 5" |\
 JDC -d -m -i -L0,100 -Am10 -ff18.8 -F'(i4,2i3),10' |\
 JDC -j -m -i -K32 -F'(f18.0),35' -k3 -:
55930.00000000  -0.5 12 90 0 5  2012-01-04 00:00:00
55930.00694444  -0.5 12 90 0 5  2012-01-04 00:10:00
55930.01388889  -0.5 12 90 0 5  2012-01-04 00:20:00
55930.02083333  -0.5 12 90 0 5  2012-01-04 00:30:00
55930.02777778  -0.5 12 90 0 5  2012-01-04 00:40:00
55930.03472222  -0.5 12 90 0 5  2012-01-04 00:50:00
55930.04166667  -0.5 12 90 0 5  2012-01-04 01:00:00
55930.04861111  -0.5 12 90 0 5  2012-01-04 01:10:00
...


# A daily table
$ awk -v f=$from -v t=$to \
  'BEGIN{for(y=f;y<t;y+=1/365.25){printf "%12.6f %12.6f\n",y,y}}' |\
   JDC -i -n -F,12 -ji


# Get the file for the next day
$ nextday d/G1_gar_100331-1s.mc
d/G1_gar_100401-1s.mc

See how ~/bin/nextday makes use of JDC