First make sure that the OFF_SWITCH_DAT is being correctly loaded as a numeric date value. The easiest way to check is to create a list box for OFF_SWITCH_DAT and seeing whether the dates align to the left (string) or right (numeric date), or a mix.
Then you can get a Week number with Week(OFF_SWITCH_DAT), or a week/year name with WeekName(OFF_SWITCH_DAT).
Logic will get you from a to b. Imagination will take you everywhere. - A Einstein
As suggested above the most likely reason is that the timestamp does not have the expected underlying numerical value.
If the incoming format is the same as your expected format in the app, I would suggest that you setup the app format variables to match the expected output. This should minimize your need for additional formatting.
From your example format string, I noticed that you use capital letters for the time. Try altering the format variables to lower case for the time portion. For consistency, make sure all three variables are aligned format wise.