In newer versions of Excel, Excel reformats the capitalized version to
lowercase, which is why David said it worked for him. After you questioned
the capitalized versus lowercase, I took a peak. My Excel 2003 reformatted
to yyyymmdd:hhmm

From Help - format  codes:

Days, months, and years If you use "m" immediately after the "h" or "hh"
code or immediately before the "ss" code, Microsoft Excel displays minutes
instead of the month.

To display  Use this code
Months as 1-12 m
Months as 01-12 mm
Months as Jan-Dec mmm
Months as January-December mmmm
Months as the first letter of the month mmmmm
Days as 1-31 d
Days as 01-31 dd
Days as Sun-Sat ddd
Days as Sunday-Saturday dddd
Years as 00-99 yy
Years as 1900-9999 yyyy

Hours, minutes, and seconds To display Use this code
Hours as 0-23 H
Hours as 00-23 hh
Minutes as 0-59 m
Minutes as 00-59 mm
Seconds as 0-59 s
Seconds as 00-59 ss
Hours as 4 AM h AM/PM
Time as 4:36 PM h:mm AM/PM
Time as 4:36:03 P h:mm:ss A/P
Elapsed time  in hours; for example, 25.02 [h]:mm
Elapsed time in minutes; for example, 63:46 [mm]:ss
Elapsed time in seconds [ss]
Fractions of a second h:mm:ss.00

AM and PM If the format contains an AM or PM, the hour is based on the
12-hour clock, where "AM" or "A" indicates times from midnight until noon
and "PM" or "P" indicates times from noon until midnight. Otherwise, the
hour is based on the 24-hour clock. The "m" or "mm" code must appear
immediately after the "h" or "hh" code or immediately before the "ss" code;
otherwise, Microsoft Excel displays the month instead of minutes.