This is super late, but hopefully its helpful for new answer seekers. I answered a similar question here

with a somewhat new method in pandas:

pandas.merge_asof()

The parameters of interest for you would be direction,tolerance,left_on, and right_on

Building off @hernamesbarbara answer & data:

data

a = """timepoint,measure
2014-01-01 00:00:00,78
2014-01-02 00:00:00,29
2014-01-03 00:00:00,5
2014-01-04 00:00:00,73
2014-01-05 00:00:00,40
2014-01-06 00:00:00,45
2014-01-07 00:00:00,48
2014-01-08 00:00:00,2
2014-01-09 00:00:00,96
2014-01-10 00:00:00,82
2014-01-11 00:00:00,61
2014-01-12 00:00:00,68
2014-01-13 00:00:00,8
2014-01-14 00:00:00,94
2014-01-15 00:00:00,16
2014-01-16 00:00:00,31
2014-01-17 00:00:00,10
2014-01-18 00:00:00,34
2014-01-19 00:00:00,27
2014-01-20 00:00:00,58
2014-01-21 00:00:00,90
2014-01-22 00:00:00,41
2014-01-23 00:00:00,97
2014-01-24 00:00:00,7
2014-01-25 00:00:00,86
2014-01-26 00:00:00,62
2014-01-27 00:00:00,91
2014-01-28 00:00:00,0
2014-01-29 00:00:00,73
2014-01-30 00:00:00,22
2014-01-31 00:00:00,43
2014-02-01 00:00:00,87
2014-02-02 00:00:00,56
2014-02-03 00:00:00,45
2014-02-04 00:00:00,25
2014-02-05 00:00:00,92
2014-02-06 00:00:00,83
2014-02-07 00:00:00,13
2014-02-08 00:00:00,50
2014-02-09 00:00:00,48
2014-02-10 00:00:00,78"""

b = """timepoint,measure
2014-01-01 00:00:00,78
2014-01-08 00:00:00,29
2014-01-15 00:00:00,5
2014-01-22 00:00:00,73
2014-01-29 00:00:00,40
2014-02-05 00:00:00,45
2014-02-12 00:00:00,48
2014-02-19 00:00:00,2
2014-02-26 00:00:00,96
2014-03-05 00:00:00,82
2014-03-12 00:00:00,61
2014-03-19 00:00:00,68
2014-03-26 00:00:00,8
2014-04-02 00:00:00,94
"""

solution

import pandas as pd
from pandas import read_csv
from io import StringIO

df1 = pd.read_csv(StringIO(a), parse_dates=['timepoint'])
df2 = pd.read_csv(StringIO(b), parse_dates=['timepoint'])



df1['timepoint'] = pd.to_datetime(df1['timepoint'])
df2['timepoint'] = pd.to_datetime(df2['timepoint'])

# converting this to the index so we can preserve the date_start_time columns so you can validate the merging logic
df1.index = df1['timepoint']
df2.index = df2['timepoint']
# the magic happens below, check the direction and tolerance arguments
# if you want you can make a maximum tolerance on which to merge data
tol = pd.Timedelta('3 day')
df3 = pd.merge_asof(left=df1,right=df2,right_index=True,left_index=True,direction='nearest',tolerance=tol)

output

df3.head()

    timepoint_x measure_x   timepoint_y measure_y
timepoint               
2014-01-01  2014-01-01  78  2014-01-01  78
2014-01-02  2014-01-02  29  2014-01-01  78
2014-01-03  2014-01-03  5   2014-01-01  78
2014-01-04  2014-01-04  73  2014-01-01  78
2014-01-05  2014-01-05  40  2014-01-08  29