For a missionary, time isn't measured so much in months as it is in weeks. Depending on what language the missionary is speaking, their time at the MTC is 2 weeks, 6 weeks, or 9 weeks. When they get to the field, a transfer period is 6 weeks: Missionaries come and go at 6-week intervals. Because of this, a mission is usually not exactly 18 months or 24 months.
Supposing a sister missionary started on February 8th, 2012 and was at the MTC for two weeks. She would arrive in her mission around February 22nd. The following would be transfer dates in her mission:
22 February 2012
4 April 2012
16 May 2012
27 June 2012
8 August 2012
19 September 2012
31 October 2012
12 December 2012
23 January 2013
6 March 2013
17 April 2013
29 May 2013
10 July 2013
21 August 2013
Missionaries don't usually come home in the middle of a transfer period.
Given that 18 months from 8 February 2012 would be 8 August 2013, she would need to come home on either 10 July 2013, or 21 August 2013. Most likely she would come home on 21 August 2013, which is 13 days after her 18-month mark.
If she was at the MTC for 6 weeks, she would probably come home on 7 August 2013, which is 1 day before her 18-month mark.
If she was at the MTC for 9 weeks, she would come home on either 17 July 2013 or 28 August 2013. 17 July 2013 would be 22 days before her 18-month mark, and 28 August 2013 would be 20 days after her 18-month mark.
If she came home 20 days after her 18-month mark, it would make sense for her ward to be charged for an extra month.
Samuel Bradshaw • If you desire to serve God, you are called to the work.