Microsoft Excel supports formatting
timestamps
in a workbook as a date followed by a time with an AM or PM value, i.e., the
times are in 12-hour clock format where noon is 12:00 PM and
midnight is 12:00 AM. You can select that format for a cell or
column in a worksheet by highlighting the cell(s) or column (a
column can be selected by clicking on the letter at the top of the
column) and then clicking on **Format** then **Cells** and
then clicking on **Date ** and selecting a type of "3/14/15 1:30
PM."

If you wanted to extract just the hour from the timestamp, you can use
the `HOUR`

function, e.g. `=HOUR(A1)`

to extract the hour
value from cell A1. The hour will be displayed in 24-hour clock format, aka
military time, i.e., 7:00 AM is 7 while 7:00 PM
is 12 plus 7, i.e., 19; 12:00 AM (midnight) is 0 and 12:00 PM (noon) is 12. For
instance, give timestamps in column A with the time displayed as
AM or PM values and using the HOUR function in a formula for the
cells in column B, I would see the following:

A | B | |

1 | 7/1/17 1:30 PM | 13 |

2 | 7/7/17 7:42 AM | 7 |

3 | 7/12/17 2:40 PM | 14 |

4 | 7/13/17 4:45 PM | 16 |

5 | 7/14/17 12:00 AM | 0 |

6 | 7/15/17 8:20 AM | 8 |

7 | 7/16/17 12:00 PM | 12 |

To count the number of cells that have an AM value and the number of cells that have a PM value, you can use an array formula. Again, assuming that column A holds the time stamps and as in the example above, cells A1 through A7 contain the timestamp entries, I could use the following formulas:

AM | =SUM((HOUR(A1:A7)<12)*1) |

PM | =SUM((HOUR(A1:A7)>11)*1) |

Note: you have to hit *control-shift-enter*, i.e., hit the
*control*, *shift*, and *enter* keys simultanesouly,
after typing the formulas to make them array formulas. When you do so,
you will see the formulas get
curly brackets, aka braces, around them as shown below:

AM | {=SUM((HOUR(A1:A7)<12)*1)} |

PM | {=SUM((HOUR(A1:A7)>11)*1)} |

For the example above, you would see the following:

Example workbook: Count_By_AM_PM.xlsx

References: