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Adapted from m:Template talk:YMD2MJD
Code Result MJD JD REXX
{{JULIANDAY|0000|03|01|0}} = 1721119.5 -678881 1721119.5 -306
{{JULIANDAY|0001|01|31|0}} = 1721455.5 -678545 1721455.5 30
{{JULIANDAY|0001|02|01|0}} = 1721456.5 -678544 1721456.5 31
{{JULIANDAY|0001|02|28|0}} = 1721483.5 -678517 1721483.5 58
{{JULIANDAY|0001|03|01|0}} = 1721484.5 -678516 1721484.5 59
{{JULIANDAY|0001|08|31|0}} = 1721667.5 -678333 1721667.5 242
{{JULIANDAY|0001|09|01|0}} = 1721668.5 -678332 1721668.5 243
{{JULIANDAY|0004|02|29|0}} = 1722579.5 -677421 1722579.5 1154
{{JULIANDAY|0004|03|01|0}} = 1722580.5 -677420 1722580.5 1155
{{JULIANDAY|1259|03|02|0}} = 2180960.5 -219040 2180960.5 459535
{{JULIANDAY|1516|12|30|0}} = 2275130.5 -124870 2275130.5 553705
{{JULIANDAY|1582|10|15|0}} = 2299160.5 -100840 2299160.5 577735
{{JULIANDAY|1752|09|14|0}} = 2361221.5 -38779 2361221.5 639796
{{JULIANDAY|1858|11|17|0}} = 2400000.5 0 2400000.5 678575
{{JULIANDAY|1895|02|28|0}} = 2413252.5 13252 2413252.5 691827
{{JULIANDAY|1895|03|01|0}} = 2413253.5 13253 2413253.5 691828
{{JULIANDAY|1896|02|29|0}} = 2413618.5 13618 2413618.5 692193
{{JULIANDAY|1896|03|01|0}} = 2413619.5 13619 2413619.5 692194
{{JULIANDAY|1900|02|28|0}} = 2415078.5 15078 2415078.5 693653
{{JULIANDAY|1900|03|01|0}} = 2415079.5 15079 2415079.5 693654
{{JULIANDAY|1970|01|01|0}} = 2440587.5 40587 2440587.5 719162
{{JULIANDAY|2000|02|29|0}} = 2451603.5 51603 2451603.5 730178
{{JULIANDAY|2000|03|01|0}} = 2451604.5 51604 2451604.5 730179
{{JULIANDAY|2738|11|28|0}} = 2721424.5 321424 2721424.5 999999
Examples adapted from Template:YMD2MJD (edit · talk · links · history) at Meta:
{{JULIANDAY|1900|02|28}} = 2415079, last day of February 1900
{{JULIANDAY|1900|02|29}} = 2415080, bogey but okay
{{JULIANDAY|1900|03|01}} = 2415080, no leap year
{{JULIANDAY|1900|02|30}} = 2415081, bogey but okay
{{JULIANDAY|1900|03|02}} = 2415081
{{JULIANDAY|2000|02|29}} = 2451604, last day of February 2000
{{JULIANDAY|2000|02|30}} = 2451605, bogey but okay
{{JULIANDAY|2000|03|01}} = 2451605, leap year
{{JULIANDAY|2000|02|31}} = 2451606, bogey but okay
{{JULIANDAY|2000|03|02}} = 2451606
{{JULIANDAY|2024|11|28}} = 2460643

Sandbox

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Adapted from m:Template talk:YMD2MJD
Code Result MJD JD REXX
{{JULIANDAY|0000|03|01|0}} = 1721119.5 -678881 1721119.5 -306
{{JULIANDAY|0001|01|31|0}} = 1721455.5 -678545 1721455.5 30
{{JULIANDAY|0001|02|01|0}} = 1721456.5 -678544 1721456.5 31
{{JULIANDAY|0001|02|28|0}} = 1721483.5 -678517 1721483.5 58
{{JULIANDAY|0001|03|01|0}} = 1721484.5 -678516 1721484.5 59
{{JULIANDAY|0001|08|31|0}} = 1721667.5 -678333 1721667.5 242
{{JULIANDAY|0001|09|01|0}} = 1721668.5 -678332 1721668.5 243
{{JULIANDAY|0004|02|29|0}} = 1722579.5 -677421 1722579.5 1154
{{JULIANDAY|0004|03|01|0}} = 1722580.5 -677420 1722580.5 1155
{{JULIANDAY|1259|03|02|0}} = 2180960.5 -219040 2180960.5 459535
{{JULIANDAY|1516|12|30|0}} = 2275130.5 -124870 2275130.5 553705
{{JULIANDAY|1582|10|15|0}} = 2299160.5 -100840 2299160.5 577735
{{JULIANDAY|1752|09|14|0}} = 2361221.5 -38779 2361221.5 639796
{{JULIANDAY|1858|11|17|0}} = 2400000.5 0 2400000.5 678575
{{JULIANDAY|1895|02|28|0}} = 2413252.5 13252 2413252.5 691827
{{JULIANDAY|1895|03|01|0}} = 2413253.5 13253 2413253.5 691828
{{JULIANDAY|1896|02|29|0}} = 2413618.5 13618 2413618.5 692193
{{JULIANDAY|1896|03|01|0}} = 2413619.5 13619 2413619.5 692194
{{JULIANDAY|1900|02|28|0}} = 2415078.5 15078 2415078.5 693653
{{JULIANDAY|1900|03|01|0}} = 2415079.5 15079 2415079.5 693654
{{JULIANDAY|1970|01|01|0}} = 2440587.5 40587 2440587.5 719162
{{JULIANDAY|2000|02|29|0}} = 2451603.5 51603 2451603.5 730178
{{JULIANDAY|2000|03|01|0}} = 2451604.5 51604 2451604.5 730179
{{JULIANDAY|2738|11|28|0}} = 2721424.5 321424 2721424.5 999999
Examples adapted from Template:YMD2MJD (edit · talk · links · history) at Meta:
{{JULIANDAY|1900|02|28}} = 2415079, last day of February 1900
{{JULIANDAY|1900|02|29}} = Error: Need valid year|month|day or "currentdate", bogey but okay
{{JULIANDAY|1900|03|01}} = 2415080, no leap year
{{JULIANDAY|1900|02|30}} = Error: Need valid year|month|day or "currentdate", bogey but okay
{{JULIANDAY|1900|03|02}} = 2415081
{{JULIANDAY|2000|02|29}} = 2451604, last day of February 2000
{{JULIANDAY|2000|02|30}} = Error: Need valid year|month|day or "currentdate", bogey but okay
{{JULIANDAY|2000|03|01}} = 2451605, leap year
{{JULIANDAY|2000|02|31}} = Error: Need valid year|month|day or "currentdate", bogey but okay
{{JULIANDAY|2000|03|02}} = 2451606
{{JULIANDAY|2024|11|28}} = 2460643
Added in 2018
{{JULIANDAY|1900|02}} = 2415052, should work
{{JULIANDAY|1900|02}} = Error: Need valid year|month|day or "currentdate", should work
{{JULIANDAY|1900}} = 2415021, just year
{{JULIANDAY|1900}} = Error: Need valid year|month|day or "currentdate", just year