I'm trying to compare row data over time intervals rather than row iterations. i.e. I want to compare a value with it's comparable value from 'X' mins ago.

Each row in my dataframe is not a standardized time increment. 

Specifically, I would like to compare the `heading` value in this subset dataframe with the one 2 mins ago for example.

I have tried a few methods such as timedeltas, and .shift() method, but so far no joy and I'm confusing myself. Any thoughts, or help would be appreciated.

```
index                heading               times
2015-12-09 03:00:01      NaN 2015-12-09 03:00:01
2015-12-09 03:01:07   231.12 2015-12-09 03:01:07
2015-12-09 03:01:08     0.00 2015-12-09 03:01:08
2015-12-09 03:01:10    90.00 2015-12-09 03:01:10
2015-12-09 03:01:15    90.00 2015-12-09 03:01:15
2015-12-09 03:02:22   149.23 2015-12-09 03:02:22
2015-12-09 03:02:25     0.00 2015-12-09 03:02:25
2015-12-09 03:02:32   270.00 2015-12-09 03:02:32
2015-12-09 03:02:40      NaN 2015-12-09 03:02:40
2015-12-09 03:02:42    90.00 2015-12-09 03:02:42
2015-12-09 03:02:48   270.00 2015-12-09 03:02:48
2015-12-09 03:03:15     9.39 2015-12-09 03:03:15
2015-12-09 03:03:17   210.77 2015-12-09 03:03:17
2015-12-09 03:03:35   153.61 2015-12-09 03:03:35
2015-12-09 03:03:39    90.00 2015-12-09 03:03:39
2015-12-09 03:03:40   263.84 2015-12-09 03:03:40
2015-12-09 03:03:46   351.30 2015-12-09 03:03:46
2015-12-09 03:03:48   270.00 2015-12-09 03:03:48
2015-12-09 03:03:50   267.69 2015-12-09 03:03:50
2015-12-09 03:03:51   270.00 2015-12-09 03:03:51
2015-12-09 03:04:10   205.03 2015-12-09 03:04:10
2015-12-09 03:04:11    90.00 2015-12-09 03:04:11
2015-12-09 03:04:12   270.00 2015-12-09 03:04:12
2015-12-09 03:04:18      NaN 2015-12-09 03:04:18
2015-12-09 03:04:24     0.00 2015-12-09 03:04:24

```