Database
[{"REQUESTTIMESTAMP":"26-JUN-19 01.34.10.095000000 AM"},{"REQUESTTIMESTAMP":"26-JUN-19 11.34.10.095000000 AM"},{"REQUESTTIMESTAMP":"26-JUN-19 12.34.10.095000000 AM"},{"REQUESTTIMESTAMP":"26-JUN-19 01.34.10.095000000 PM"}]
Query
db.collection.aggregate([{$addFields:{hour:{$let:{vars:{hour:{$slice:[{$split:["$REQUESTTIMESTAMP"," "]},1,2]}},in:{$cond:[{$in:["AM","$$hour"]},{$toInt:{$substr:[{$arrayElemAt:["$$hour",0]},0,2]}},{$add:[{$toInt:{$substr:[{$arrayElemAt:["$$hour",0]},0,2]}},12]}]}}}}}])