http://developer.gnome.org/doc/tutorials/gnome-i18n/developer.html#use-comments

add the following:

/* Translator: The first %s refers to the base, which would be actions like 
 * "Play a Sound". Second %s refers to the duration string e.g:"15 minutes" */
#: ../calendar/gui/e-alarm-list.c:497
#, c-format
msgid "%s %s before the end of the appointment"

/* Translator: The first %s refers to the base, which would be actions like 
 * "Play a Sound". Second %s refers to the duration string e.g:"15 minutes" */
#: ../calendar/gui/e-alarm-list.c:500
#, c-format
msgid "%s %s after the end of the appointment"

/* Translator: The %s refers to the base, which would be actions like 
 * "Play a sound" */ 
#: ../calendar/gui/e-alarm-list.c:505
#, c-format
msgid "%s at the end of the appointment"

/* Translator: The first %s refers to the base, which would be actions like 
 * "Play a Sound". Second %s is an absolute time */
#: ../calendar/gui/e-alarm-list.c:527
#, c-format
msgid "%s at %s"


/* Translator: The %s refers to the base, which would be actions like 
 * "Play a sound". "Trigger types" are absolute or relative dates */ 
#: ../calendar/gui/e-alarm-list.c:533
#, c-format
msgid "%s for an unknown trigger type"


also, the following strings have been translated WRONGLY in the german po file because of missing translator comments:

#: ../calendar/gui/e-alarm-list.c:395
#, c-format
msgid "%d day"
msgid_plural "%d days"

#: ../calendar/gui/e-alarm-list.c:400
#, c-format
msgid "%d week"
msgid_plural "%d weeks"

#: ../calendar/gui/e-alarm-list.c:405
#, c-format
msgid "%d hour"
msgid_plural "%d hours"

#: ../calendar/gui/e-alarm-list.c:410
#, c-format
msgid "%d minute"
msgid_plural "%d minutes"

#: ../calendar/gui/e-alarm-list.c:415
#, c-format
msgid "%d second"
msgid_plural "%d seconds"


note that adding comments to translatable strings must not be announced according to http://mail.gnome.org/archives/desktop-devel-list/2005-February/msg00092.html.